Userboot's copy of the libstand Makefile had more extensive changes compared to the one in sys/boot/libstand32, but it turns out these are not intentional and we can just include lib/libstand/Makefile as done for libstand32 in rS293040.
(That is, I believe they've just unintentionally diverged over time, and that we can just .include lib/libstand/Makefile. One thing I'm not quite sure about is the use of bsd.own.mk / src.opts.mk. @grehan, can you comment on these items?)