libstand is built in three places (lib/libstand, sys/boot/libstand32, and sys/boot/userboot/libstand). Reduce Makefile duplication by .including libstand/Makefile from sys/boot/libstand32/Makefile. The one in sys/boot/userboot/libstand will be addressed later, as it contains additional differences to be handled still.
This change also switches libstand32 to use the new uuid_from_string and uuid_to_string, missed in rS292473.