kernel: mtdsplit: add missing digest field to Seama header

kernel: mtdsplit: add missing digest field to Seama header

Seama format has 2 similar headers: container (seal) header and entity
header. The first one has size always set to 0 and doesn't contain MD5
digest.

When dealing with Seama on a flash we deal directly with an entity. You
can see mtdsplit_parse_seama reads from offset 0 and expects entity to
be there. Seama container is used by bootloader / interface only which
extract entity out of it and flash it.

That said we should fix our header struct. This is important as we
calculate possible rootfs offset assuming it may be placed right after
Seama entity. So far calculate offset was always 16B too low.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@48754 3c298f89-4303-0410-b956-a3cf2f4a3e73