Restore /boot/efi to mtree.

Instead of whether /boot/efi exists, which it now always does, including
on systems that don't and can't use EFI, use whether /boot/efi is
present in fstab to signal to the installer that it is a valid ESP and
should be configured. This has essentially the same semantics, but allows
/boot/efi to be created unconditionally.

Reviewed by: bdragon, imp
Tested by: bdragon (ppc64)
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D29068