disk: part_efi: parse and store partition UUID

Each EFI partition table entry contains a UUID. Extend U-Boot's struct
disk_partition to be able to store this information, and modify
get_partition_info_efi() to fill it in.

The implementation of uuid_string() was derived from the Linux kernel,
tag v3.6-rc4 file lib/vsprintf.c function uuid_string().

Signed-off-by: Stephen Warren <swarren@nvidia.com>