Add support for verifying an image by use of the imd checksum.
If the checksum that is saved in the image and the one that is
calculated over the image differ, barebox_update aborts.

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