kboot: Use standard set_currdev

Use the standard set_currdev instead of the (now very old) copy of
setting currdev and loaddev directly. We do this only when we don't go
find the ZFS pool to boot from.

Sponsored by: Netflix
Reviewed by: kevans
Differential Revision: https://reviews.freebsd.org/D38012

(cherry picked from commit eb1795782cfc43523345f78737cc22277cea5056)