All images generated by mkimage are now always validated.
This change broke our downstream support for setting a custom value for
IH_MAGIC (mkimage -M ...). Make sure also plain legacy kernel images
with custom value set for IH_MAGIC validate correctly.
Fixes: fa9895ee5b ("tools: mkimage: update to U-Boot release 2022.07")
Signed-off-by: Daniel Golle <daniel@makrotopia.org>