stand/loader.efi: read zfs bootonce attribute before checking currdev

First check if bootonce is configured and if it is, then change currdev
accordingly and after that do the sanity check. This fixes boot in a
situation when ZFS pool doesn't have the "bootfs" property, but has
bootonce attribute set. A strange, but legitimate case.

Reviewed by: tsoome, imp
Differential Revision: https://reviews.freebsd.org/D40388