part: extract MBR signature from partitions

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 <[email protected]>
[separated from efi_loader part, and fixed build-errors for non-
 CONFIG_EFI_PARTITION case]
Signed-off-by: Rob Clark <[email protected]>
Signed-off-by: Alexander Graf <[email protected]>