Make the root-on-zfs part of the installer warn a user who booted the installer via UEFI that we do not support booting ZFS via UEFI yet

PR:		193595
Approved by:	nwhitehorn
MFC after:	5 days
Sponsored by:	ScaleEngine Inc.
CR:		https://reviews.freebsd.org/D782


git-svn-id: svn+ssh://svn.freebsd.org/base/head@271563 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f