Reduce libstand Makefile duplication

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.

sys/boot/userboot/libstand/Makefile will be addressed later, as it
contains additional differences yet to be handled.

This change also switches libstand32 to use the new uuid_from_string
and uuid_to_string, which was not included in r292473.

Reviewed by: jhb
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D4747