The bootrom only reads an image if the correct checksum is present in the
header. The calculation is pretty simple:
sum over all words from 0x20 to 0x44
Two of this words are the image length. That is why the checksum can not be
calculated until barebox_image_size is known.
The easiest solution is a program that has to be run after make.

Signed-off-by: Steffen Trumtrar <s.trumtrar@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>