stand/i386: 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/D37341

(cherry picked from commit 641a0617e8d060c3c5fb997cee9e2b7fb40d9306)