This commits adds initial support for the Kinetis Freedom-K28F board.  Not you ready for prime time.

Squashed commit of the following:

    arch/arm/src/kinetis:  A fixes from first (unsuccessful) attempt to build the Freedom-K28F configuration.
    configs/freedom-k28f:  Add basic board support for Freedom-K28F.  Initial commit is basically just the Teensy-3.x brought up to date and with name changes.