stand/boot1.efi: use the bootonce dataset as root dataset

Before this change we would only pass the bootonce dataset name
to the environment for the next loader, while actually reading
the next stage loader from the 'bootfs' dataset, not the bootonce
dataset.

Another problem fixed by this change is a boot from a configuration
when bootonce attribute is present, but 'bootfs' property is not set.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D40389