Allow build profiles to omit language entries

[lfdebrux]

Allow toolchain build profiles to omit language entries

Description

Currently the prepare_toolchain function in build_api.py expects all profiles to contain all language keys. When writing a toolchain profile to overlay on the default profiles this can cause unexpected behaviour: ie. if you don't have an entry for the key there is an opaque error.

The workaround is to have a profile.json file that looks like this:

"TOOLCHAIN": {
    "common": [],
    "asm": [],
    "c": [],
    "cxx": ["-flag"],
    "ld": []
}

but it would be nice to allow this

"TOOLCHAIN": {
    "cxx": ["-flag"],
}

This pull request allows this by modifying the code that gets the language key from the dictionary.

Pull request type

• Fix
• Refactor
• New target
• Feature
• Breaking change

[theotherjimmy]

Good idea!

[theotherjimmy]

/morph build

[mbed-ci]

Build : SUCCESS

Build number : 1345
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/6271/

Triggering tests

/morph test
/morph uvisor-test
/morph export-build
/morph mbed2-build

[mbed-ci]

Exporter Build : SUCCESS

Build number : 1001
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/6271/

[mbed-ci]

Test : SUCCESS

Build number : 1128
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/6271/1128

[cmonr]

Hi @lfdebrux, would you mind signing the CLA: https://developer.mbed.org/contributor_agreement/ ?

[studavekar]

/morph mbed2-build

[lfdebrux]

@cmonr accepted.