Mbed OS PR #12278 breaks compilation

[marcuschangarm]

Mbed OS renamed target.default_lib to target.c_lib in PR ARMmbed/mbed-os#12278 which breaks the current configuration files.

@SeppoTakalo I would recommend pinning Mbed OS to a version to your liking.

[SeppoTakalo]

Fixed in #61

So master branch works against Mbed OS master. Release branch work against 5.15 release.