Add a pre-finalise.d phase

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

diskimage_builder/lib/disk-image-create

@@ -486,6 +486,7 @@ sudo cp -ra ${TMP_BUILD_DIR}/built/* $TMP_BUILD_DIR/mnt
 sudo rm -fr ${TMP_BUILD_DIR}/built/*
 
 mount_proc_dev_sys
+run_d pre-finalise
 run_d_in_target finalise
 finalise_base
 

doc/source/developer/developing_elements.rst

@@ -137,6 +137,16 @@ The phases are:
 
   * outputs: ``$IMAGE_BLOCK_DEVICE={path}``
 
+``pre-finalise.d``
+
+  Final tuning of the root filesystem, outside the chroot.  Filesystem
+  content has been copied into the final file system which is rooted
+  at ``$TMP_BUILD_DIR/mnt``.  You might do things like re-mount a
+  cache directory that was used during the build in this phase (with
+  subsequent unmount in ``cleanup.d``).
+
+  * runs: **outside chroot**
+
 ``finalise.d``
   Perform final tuning of the root filesystem. Runs in a chroot after the root
   filesystem content has been copied into the mounted filesystem: this is an

releasenotes/notes/pre-finalise-stage-574ae7886274bcba.yaml

@@ -0,0 +1,8 @@
+---
+features:
+  - |
+    A ``pre-finalise`` stage is added that runs outside the chroot and
+    before the ``finalise`` stage.  For example, this may be useful
+    for mounting external resources that are used inside the chroot
+    during the ``finalise`` stage, but do not need to be distributed
+    in the final image such as build-time caches.