sched/init/nx_start.c: Add support for CONFIG_BOARD_DRIVER_INITIALIZE. If this option is selected then nx_start() will call a board-provided function board_driver_initialize() immediately after calling up_initialize().
Rename CONFIG_BOARD_INITIALIZE to CONFIG_BOARD_LATE_INITIALIZE.
Raname board_initialize() to board_late_initialize()
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.