configs/freedom-k28f: Add button and LED support.
arch/arm/src/kinetis: Fix a couple of additional compile problems.
arch/arm/src/kinetis/chip and configs/freedom-k28f: Add things needed for a successful build. Still need board LED definitions.
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.