Instead of passing the offset to the fix_size tool check the image to
fixup for a valid header so that only recognized files are fixed up.
This makes the usage of this tool safer.

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

this will allow to write the size of barebox at an offset of the binary

this is needed for ARM when using relocated binary

Signed-off-by: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>