gcc is more widely used in the community due to being open source and free...
changed the default compiler to suit the common case.
Previously the default target in .yotta.json was targetting
a namespace that did exist originally, but no longer does.
The default target is now bbc-microbit-classic-armcc.
This commit adds the bbc-microbit-armcc target, and
also includes the microbit dal, showing how the microbit
can be compiled using yotta.