This renames BOARD_LATE_INIT to CONFIG_BOARD_LATE_INIT.
Along the way it removes some leftover

 #define BOARD_LATE_INIT		1

and adds some basic documentation for board specific
callbacks in README.
Signed-off-by: NHelmut Raiger <helmut.raiger@hale.at>
Acked-by: NStefano Babic <sbabic@denx.de>