stand: Create common gen_setcurrdev and replace code

Replace 4 identical copies of *_setcurrdev with gen_setcurrdev to avoid
having to create a 5th copy. uboot_setcurrdev is actually different and
needs to remain separate (even though it's quite similar).

Sponsored by: Netflix
Reviewed by: fuz@fuz.su, kevans
Differential Revision: https://reviews.freebsd.org/D38003