feat(partition): copy the partition GUID into the partition structure

Copy the UniquePartitionGUID member of a GPT partition entry into the
partition_entry structure. This GUID is subsequently used to identify
the image to boot on a platform which supports multiple partitions of
firmware components using the FWU metadata structure.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Change-Id: I8b55a1ee7deb1353886fbd8ebde53055d677fee0