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()
sched/init/nx_bringup.c: Fix a naming collision.
sched/init: Rename os_start() to nx_start()
sched/init: Rename os_smp* to nx_smp*
sched/init: Rename os_bringup to nx_bringup
sched/init: rename all internal static functions to begin with nx_ vs os_