Implement uefi_rootdev

If uefi_rootdev is set in the environment, then treat it like a device
path. Convert the string to a device path and see if we can find a
device that matches. If so, use that device at our root dev no matter
what. If it's bad in any way, the boot will fail.

Reviewed by: bcran
Differential Revision: https://reviews.freebsd.org/D20016