Moved target definitions to JSON format

(long commit message ahead. Sorry about that, it can't be helped.)

This commit changs targets definition from Python to JSON format, as
part of the configuration mechanism implementation. There is a new file
under tools/ called "targets.json" which contains the target
definitions. "targets.py" remains, but becomes a wrapper on top of
"targets.json", with the same interface as before. This has the
advantage of not requiring code changes outside "targets.py".

Most of the JSON definitions of targets were automatically generated by a
script (available upon request since it doesn't make a lot of sense to
include it here), only those targets that had more than one parent in
the Python implementation were converted by hand. The target definitions
should be pretty self-explanatory. A number of things are different in
the JSON implementation (this is just a summary, a separate commit that
documents how to write target definitions will follow later):

- "program_cycle_s" is now a value (as opposed to a function in the
Python implementation), since it only returned a number in all the
Python target implementations. The main definition that actually contains
some code (in class "Target") remains in target.py
- array values in "macros" and "extra_labels" can be modified
dynamically. Values can be added using "macros_add" and
"extra_labels_add" or removed using "macros_remove" and
"extra_labels_remove". This mechanism is available for all attributes
with a list type, but it's currently enabled only for "macros" and
"extra_labels" to keep things simple.
- "init_hooks"/"binary_hook" are now implemented in terms of a single
JSON key valled "post_binary_hook". The corresponding code is also in
"targets.py", under the various TargetCode classes (see for example
LPC4088Code in targets.py).

Just like in the Python implementation, a target can inherit from zero,
one or more targets. The resolution order for the target's attributes
follows the one used by the Python code (I used
http://makina-corpus.com/blog/metier/2014/python-tutorial-understanding-python-mro-class-search-path
as a reference for the implementation of resolution order).

This is obviously a very dangerous commit, since it affects all targets.
Unfortunately, since mbed-os only compiles for K64F, I didn't have many
chances to try to compile mbed-os with different targets. I did it for
K64F in different scenarios (pretending that it has hooks, for example),
but there's definitely a lot more to be done there. The threaded blinky
example for K64F compiles and works.

I tried to test in a different way: I wrote a script that imports the
old (Python) and the new (JSON) implementations and verifies that the
attributes in the old implementations exist and have the same values
in the new implementations (it also verifies that the attribute
resolution order is the same in the two implementations). If you're
interested, the script is here:

https://gist.github.com/bogdanm/4caf3a11613dcec834b05d5ab87c65dc

And the results of running the script are below (note that the script
outputs only the target names that were found to be problematic):

F51_MICROBIT_BOOT:
    Resolution order is different in old and new
        old: ['NRF51_MICROBIT_BOOT', 'MCU_NRF51_16K_BOOT_S110', 'MCU_NRF51_16K_BOOT_BASE', 'MCU_NRF51_16K_BASE', 'MCU_NRF51', 'Target', 'MCU_NRF51_S110']
        new: ['NRF51_MICROBIT_BOOT', 'MCU_NRF51_16K_BOOT_S110', 'MCU_NRF51_S110', 'MCU_NRF51_16K_BOOT_BASE', 'MCU_NRF51_16K_BASE', 'MCU_NRF51', 'Target']
    'extra_labels' has different values in old and new
        old: ['NORDIC', 'MCU_NRF51', 'MCU_NRF51822', 'MCU_NORDIC_16K', 'MCU_NRF51_16K', 'MCU_NRF51_16K_BOOT', 'MCU_NRF51_16K_S110', 'NRF51_MICROBIT']
        new: ['NORDIC', 'MCU_NRF51', 'MCU_NRF51822', 'MCU_NORDIC_16K', 'MCU_NRF51_16K', 'MCU_NRF51_16K_S110', 'MCU_NRF51_16K_BOOT', 'NRF51_MICROBIT']
    'macros' has different values in old and new
        old: ['NRF51', 'TARGET_NRF51822', 'TARGET_MCU_NORDIC_16K', 'TARGET_MCU_NRF51_16K', 'TARGET_MCU_NRF51_16K_BOOT', 'TARGET_OTA_ENABLED', 'TARGET_MCU_NRF51_16K_S110', 'TARGET_NRF51_MICROBIT', 'TARGET_NRF_LFCLK_RC']
        new: ['NRF51', 'TARGET_NRF51822', 'TARGET_MCU_NORDIC_16K', 'TARGET_MCU_NRF51_16K', 'TARGET_MCU_NRF51_16K_S110', 'TARGET_MCU_NRF51_16K_BOOT', 'TARGET_OTA_ENABLED', 'TARGET_NRF51_MICROBIT', 'TARGET_NRF_LFCLK_RC']
NRF51_MICROBIT:
    Resolution order is different in old and new
        old: ['NRF51_MICROBIT', 'MCU_NRF51_16K_S110', 'MCU_NRF51_16K_BASE', 'MCU_NRF51', 'Target', 'MCU_NRF51_S110']
        new: ['NRF51_MICROBIT', 'MCU_NRF51_16K_S110', 'MCU_NRF51_S110', 'MCU_NRF51_16K_BASE', 'MCU_NRF51', 'Target']
    'extra_labels' has different values in old and new
        old: ['NORDIC', 'MCU_NRF51', 'MCU_NRF51822', 'MCU_NORDIC_16K', 'MCU_NRF51_16K', 'MCU_NRF51_16K_S110']
        new: ['NORDIC', 'MCU_NRF51', 'MCU_NRF51822', 'MCU_NRF51_16K_S110', 'MCU_NORDIC_16K', 'MCU_NRF51_16K']
    'macros' has different values in old and new
        old: ['NRF51', 'TARGET_NRF51822', 'TARGET_MCU_NORDIC_16K', 'TARGET_MCU_NRF51_16K', 'TARGET_MCU_NRF51_16K_S110', 'TARGET_NRF_LFCLK_RC']
        new: ['NRF51', 'TARGET_NRF51822', 'TARGET_MCU_NRF51_16K_S110', 'TARGET_MCU_NORDIC_16K', 'TARGET_MCU_NRF51_16K', 'TARGET_NRF_LFCLK_RC']
NRF51_MICROBIT_OTA:
    Resolution order is different in old and new
        old: ['NRF51_MICROBIT_OTA', 'MCU_NRF51_16K_OTA_S110', 'MCU_NRF51_16K_OTA_BASE', 'MCU_NRF51_16K_BASE', 'MCU_NRF51', 'Target', 'MCU_NRF51_S110']
        new: ['NRF51_MICROBIT_OTA', 'MCU_NRF51_16K_OTA_S110', 'MCU_NRF51_S110', 'MCU_NRF51_16K_OTA_BASE', 'MCU_NRF51_16K_BASE', 'MCU_NRF51', 'Target']
    'extra_labels' has different values in old and new
        old: ['NORDIC', 'MCU_NRF51', 'MCU_NRF51822', 'MCU_NORDIC_16K', 'MCU_NRF51_16K', 'MCU_NRF51_16K_OTA', 'MCU_NRF51_16K_S110', 'NRF51_MICROBIT']
        new: ['NORDIC', 'MCU_NRF51', 'MCU_NRF51822', 'MCU_NORDIC_16K', 'MCU_NRF51_16K', 'MCU_NRF51_16K_S110', 'MCU_NRF51_16K_OTA', 'NRF51_MICROBIT']
    'macros' has different values in old and new
        old: ['NRF51', 'TARGET_NRF51822', 'TARGET_MCU_NORDIC_16K', 'TARGET_MCU_NRF51_16K', 'TARGET_MCU_NRF51_16K_OTA', 'TARGET_OTA_ENABLED', 'TARGET_MCU_NRF51_16K_S110', 'TARGET_NRF51_MICROBIT', 'TARGET_NRF_LFCLK_RC']
        new: ['NRF51', 'TARGET_NRF51822', 'TARGET_MCU_NORDIC_16K', 'TARGET_MCU_NRF51_16K', 'TARGET_MCU_NRF51_16K_S110', 'TARGET_MCU_NRF51_16K_OTA', 'TARGET_OTA_ENABLED', 'TARGET_NRF51_MICROBIT', 'TARGET_NRF_LFCLK_RC']
NOT OK: ['NRF51_MICROBIT', 'NRF51_MICROBIT_BOOT', 'NRF51_MICROBIT_OTA']

The reasons for the above output are subtle and related to the
extremely weird way in which we defined target data in the old
implementation: we used both class attributes and instance attributes.
This can complicate resolution order quite a bit and those two levels
don't exist in JSON: there's only one attribute type (equivalent to
Python's instance attributes). To make that work, I had to change the
inheritance order of the above targets (that use multiple inheritance)
which in turn changed the order of some macros and extra_labels (and of
course the resolution order). No harm done: the values are the same,
only their ordering is different. I don't believe this causes any
problems for 'extra_labels' and 'macros'.

This method of testing has its limitations though; in particular, it
can't test the hooks. I'm opened to ideas about how to test this better,
but I think that we need to remember that this commit might break some
targets and keep an eye out for "weird errors" in the future.

Author: Bogdan Marinescu