NAND: move board_nand_init to nand.h

Rather than putting the function prototype for board_nand_init() in the one
place where it gets called, put it into nand.h so that every place that also
defines it gets the prototype.  Otherwise, errors can go silently unnoticed
such as using the wrong return value (void rather than int) when defining
the function.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Scott Wood <scottwood@freescale.com>