spl, nand: add option to boot raw u-boot.bin image only

enable to boot only a raw u-boot.bin image from nand with the
CONFIG_SPL_NAND_RAW_ONLY define. This option saves space on
boards where spl space is low.

Signed-off-by: Heiko Schocher <hs@denx.de>
Reviewed-by: Andreas Bießmann <andreas.devel@googlemail.com>
Reviewed-by: Bo Shen <voice.shen@atmel.com>
Signed-off-by: Andreas Bießmann <andreas.devel@googlemail.com>