In exploring Gentoo caching, it was realised that we have no way to
bind mount the cache into the finalised image for the finalise.d
phases.
By adding a pre-finalise.d phase that runs outside the chroot, we can
mount outside things into the hierarchy at $TMP_BUILD_DIR/mnt which
are then seen by the in-chroot finalise.d phase.
This is similar to the pre-install phase
Change-Id: I9d782994843383ddf90f62c40498af9925fd9558
