tools: kwbimage: Validate data checksum of v1 images

The data part of v1 images contains 32-bit checksum after the data.
Validate whether this checksum is correct.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Stefan Roese <sr@denx.de>
Reviewed-by: Chris Packham <judge.packham@gmail.com>
Tested-by: Chris Packham <judge.packham@gmail.com>