Catch the EFI loader up to the latest ZFS Boot Environment Menu features
MFC: r294072
Move init_zfs_bootenv to sys/boot/zfs/zfs.c instead of having a copy in each loader
MFC: r294073
Connect the ZFS boot environment menu to the UEFI loader
MFC: r295357
Do not set vfs.root.mountfrom unnecessarily when initializing ZFS BE menu
Approved by: re (marius)
Relnotes: yes
Sponsored by: ScaleEngine Inc.