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()
many locations: Change occurences of open() followed by file_detach() to file_open(). Change most non-controversion calls to open() to nx_open().
fs/inode/fs_fileopen.c: Flesh out file_open() with some interim, placeholder logic.
fs/inode/fs_fileopen.c: Add a framework for a file_open() implementation (no real logic in place yet).
fs/vfs/fs_open.c: Add nx_open() which is the same as open() except that it does not create a cancellation point nor does it modify the errno variable.
