tools: kwboot: Recalculate 4-byte data checksum after injecting baudrate code

If data part of image is modified, update 4-byte data checksum.

It looks like A385 BootROM does not verify this checksum for image
loaded via UART, but we do not know if other BootROMs are also ignoring
it. It is always better to provide correct checksum.

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>