mtdsplit_uimage: more generic header verify function

Some devices have uImage headers after some extra headers (e.g. Edimax
devices). To support such cases our verify callback function should be
allowed to return header offset, not just a boolean value.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>


git-svn-id: svn://svn.openwrt.org/openwrt/trunk@44412 3c298f89-4303-0410-b956-a3cf2f4a3e73