Preparation for the next step which will allow to open
images which are not part of a configuration.

This has one change inside: We used to iterate over all
subnodes of a image expecting all of them containing a hash,
so it could happen that we check multiple hashes if more exist
or that we falsely interpret some unrelated subnode as hash node.
With this patch we expect the hash in a subnode named "hash@1"
as required by the FIT image format description.

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