tools: kwboot: Patch source address in image header

Some image types have source address in non-bytes unit; for example for
SATA images, it is in 512 B units.

We need to multiply by unit size when patching image type to UART.

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>