arm: mvebu: Check for kwbimage data checksum

Last 4 bytes of kwbimage boot image is checksum. Verify it via the new
spl_check_board_image() function which is called by U-Boot SPL after
loading kwbimage.

Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Stefan Roese <sr@denx.de>