stand/userboot: Move to using common devparse()

We no longer need to have to hand-code this for each boot loader since
devparse() handles them all with dv_parsedev().

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D37342

(cherry picked from commit 910de60a7e43c593b5126b2f1ee468c089c8116e)