Update mbed-cli to allow different configured path for mbed-os

[daid]

In not every project structure it makes sense to have the mbed-os directory at the root of your project structure.
This change allows you to configure a custom path for the mbed-os directory in your .mbed configuration.

In our case, our project calls for multiple firmware builds. But we do not want checkout mbed-os per firmware. mbed-cli allows this with the --source parameter, however, it does require that mbed-os is in the root of the project.

With this change we can setup mbed-os in a better subfolder to allow for a better project structure:

extlibs/mbed-os
module/mobuleA
module/moduleB
project/project1
project/project2

And compile those with:

mbed-cli config MBED_OS_DIR extlibs/mbed-os/
mbed-cli compile -f --profile release --source project/project1 --source module --source extlibs/mbed-os --build BUILD/project1
mbed-cli compile -f --profile release --source project/project2 --source module --source extlibs/mbed-os --build BUILD/project2

[theotherjimmy]

This is a very interesting, and quite simple, change. I'm okay with it. @screamerbg What do you think?

[0Grit]

Yes please. I'd like to use this to support adding mbed-os as a git submodule versus having mbed-cli import it.

[screamerbg]

@daid As @theotherjimmy pointed, it's a simple and non-breaking change. Thanks!

[theotherjimmy]

Looks great. Wonderful small diff!

[bmcdonnell-ionx]

Does this work with exporters?

[theotherjimmy]

@bmcdonnell-ionx This should work with all mbed verbs, export included.

[daid]

@bmcdonnell-ionx #590 is a bigger problem for complex project setups with the exporter. As this patch only helps the python tools find the actual exporter (and more things), I don't think it causes any problems. And any problems that it would cause are most likely bugs due to wrong assumptions on paths.