tools: kwbimage: Do not put final image padding to the image data size

This change allows to convert image from one format to another without need
to include unnecessary padding (e.g. when target image format has smaller
alignment requirement as source image format).

Do it by storing real image data size without padding to the kwbimage
header.

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