Add static delta superblock signature verification

Add aum_get_target_metadata to verify the superblock metadata, expect signature and commits.

Add aum_load_target_superblock to load static delta superblock, used by aum_get_target_metadata and aum_target_superblock_verify_signature.
This prevents 2 times read problem, i.e. a first read from the media device to check signature and claim static delta as a "safe" file followed by a second read to parse the superblock again. The special HW with USB media "emulation" may substitute bits to others for the second read.

Add function to load keys, shared by aum_check_signature and aum_target_superblock_verify_signature.

Signed-off-by: Frédéric Danis frederic.danis@collabora.com