tools: kwboot: Validate 4-byte image data checksum

Data part of the image contains 4-byte checksum. Validate it when
processing the image.

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