Revert "Remove support for EFM32.  Not ready to be fielded"

This reverts commit 79f13683cc25f4ebafde57c72baaf31ae8a55915.

Conflicts:
	nuttx/ChangeLog
Revert "Remove EFM32 from build system"

This reverts commit 97b98200709f76e968fe73dd5a7ed0c43e4084db.