tools: kwboot: Workaround A38x BootROM bug for images with a gap

A38x BootROM has a bug which cause that BootROM loads data part of UART
image into RAM target address increased by one byte when source address
and header size stored in the image header are not same.

Workaround this bug by completely removing a gap between header and data
part of the UART image. Without gap, this BootROM bug is not triggered.

This gap can be present in SDIO or SATA image types which have aligned
start of the data part to the media sector size. With this workaround
kwboot should be able to convert and send SDIO or SATA images for UART
booting.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Stefan Roese <sr@denx.de>