This adds a spl command to the u-boot.

Related config:
CONFIG_CMD_SPL
	activate/deactivate the command
CONFIG_CMD_SPL_NAND_OFS
	Offset in NAND to use

Signed-off-by: Simon Schwarz <simonschwarzcor@gmail.com>
Signed-off-by: Stefano Babic <sbabic@denx.de>
CC: Tom Rini <tom.rini@gmail.com>
CC: Wolfgang Denk <wd@denx.de>