tools: kwbimage: Align final UART image to 128 bytes

xmodem block size is 128 bytes, therefore it is possible to transfer only
images with size multiple of 128 bytes. kwboot automatically pads image
with zero bytes at the end to align it to 128 bytes boundary.

Do this padding when generating image to allow uploading with other xmodem
tools or older kwboot versions.

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