
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