Enable override of variant name via board.json

[frankleonrose]

Fix for issue #14.

[frankleonrose]

Replicating on local machine - if I remove my change, errors still occur.

I recommend running CircleCI on a weekly schedule in order to detect errors based on changes in dependencies.

[valeros]

Thanks a lot for the PR. The code in this repository is quite outdated so I need to update in the first place. As for your suggestion, some of the boards (e.g generic* from ststm32 platform) have default variant field reserved for Arduino framework, so that won't be that easy to use variant with mbed.

[frankleonrose]

I understand. I'm not surprised to run into ecosystem-level conflicts - hard to see at my level.

Does it make sense to use a different name? build.mbed_variant or build.mbed.variant? Or is there a whole different way this should be done?