part: extract MBR signature from partitions

EFI client programs need the signature information from the partition
table to determine the disk a partition is on, so we need to fill that
in here.

Signed-off-by: Peter Jones <pjones@redhat.com>
[separated from efi_loader part, and fixed build-errors for non-
 CONFIG_EFI_PARTITION case]
Signed-off-by: Rob Clark <robdclark@gmail.com>
Signed-off-by: Alexander Graf <agraf@suse.de>