Add app config switch

[ConorPKeegan]

Changes

• Add command line switch --app-config to specify the location of an app config file
• Specified file must be in the same format as mbed_app.json

Example

This is an example use of the switch, where test.json is in the format of a mbed_app.json file

$ mbed test --compile -m K64F -t GCC_ARM --app-config test.json

Dependencies

• This pull request depends on Add app config command line switch for test and make mbed-os#2593

[ConorPKeegan]

@mazimkhan Please can you review this

[ConorPKeegan]

@bridadan To include the new changes from ARMmbed/mbed-os#2593 (the changes in make.py), which command needs to be changed, it seems to me that it would be compile?

[bridadan]

Yup! mbed compile goes to make.py and mbed test goes to test.py (only for --compile and --compile-list)

[ConorPKeegan]

@sg- @screamerbg Can we get this merged (depends on ARMmbed/mbed-os#2593)?

[screamerbg]

LGTM