tools: kwbimage: Fix dest addr

tools: kwbimage: Fix dest addr

To enable secure boot, we need to jump back into the BootROM to continue
the SoC's boot process instead of letting the SPL load and run the main
U-Boot image.

But, since the u-boot-spl.img (including the 64 byte header) is loaded
by the SoC as the main image, we need to compensate for the header
length to get a correct entry point.

Thus, we subtract the header size from the destination address, so that
the execution address points at the actual entry point of the image.

The current boards ignore both parameters anyway, so this change shouldn't
concern them.

Signed-off-by: Mario Six <mario.six@gdsys.cc>
Reviewed-by: Stefan Roese <sr@denx.de>
Signed-off-by: Stefan Roese <sr@denx.de>