[Exporter-gcc_arm] do builds from project directory when --source is specified

[theotherjimmy]

Also, fix a few bugs along the way:

• python barf when something in features is None

[0xc0170]

cc @sg- @bridadan @bogdanm please test any example you have, using gcc_arm and report

[sg-]

I tried a few ways and still not seeing success

(workspace) C:\Development\tmp\mbed-os-example-blinky>mbed export -i gcc_arm -m K64F -n blinky

Successful exports:
  * K64F::gcc_arm       .\projectfiles\gcc_arm_K64F\blinky

(workspace) C:\Development\tmp\mbed-os-example-blinky>make .\projectfiles\gcc_arm_K64F\Makefile
make: Nothing to be done for `.\projectfiles\gcc_arm_K64F\Makefile'.

(workspace) C:\Development\tmp\mbed-os-example-blinky>make .\projectfiles\gcc_arm_K64F\Makefile all
make: Nothing to be done for `.\projectfiles\gcc_arm_K64F\Makefile'.
make: *** No rule to make target `all'.  Stop.

(workspace) C:\Development\tmp\mbed-os-example-blinky>cd projectfiles\gcc_arm_K64F

(workspace) C:\Development\tmp\mbed-os-example-blinky\projectfiles\gcc_arm_K64F>make
/usr/bin/bash: -c: line 1: syntax error: unexpected end of file
make: *** [all] Error 1

(workspace) C:\Development\tmp\mbed-os-example-blinky\projectfiles\gcc_arm_K64F>make all
/usr/bin/bash: -c: line 1: syntax error: unexpected end of file
make: *** [all] Error 1

(workspace) C:\Development\tmp\mbed-os-example-blinky\projectfiles\gcc_arm_K64F>

[0xc0170]

My feedback:

• project.py - makefile working for MBED_10
• mbed export - working for me as well

Both are executed within the exporter project (cd to the project, execute make)

Using windows, linux test is required as well

[0xc0170]

I was able to reproduce Sam's report, using cygwin under windows:

/bin/sh: -c: line 1: syntax error: unexpected end of file
Makefile:21: recipe for target 'all' failed
make: *** [all] Error 1

@theotherjimmy Please test on windows/unix :-)

[sg-]

LGTM 👍