cmdlib: consistently use overlay/ prefix for injected overlays

Like in e6b07ff ("cmdlib: rename `cosa-image-json` ostree overlay"),
we need the content set overlay and the `overrides/rootfs` overlay to
use that prefix. That's how `compose.sh` knows to pull it over 9p.

Fixes: coreos#2949
(cherry picked from commit ea7287c)

dustymabe: This cherry pick needed manual merge conflict resolution.

src/cmdlib.sh

@@ -406,12 +406,12 @@ EOF
     if [[ -d "${rootfs_overrides}" && -n $(ls -A "${rootfs_overrides}") ]]; then
         echo -n "Committing ${rootfs_overrides}... "
         touch "${overrides_active_stamp}"
-        ostree commit --repo="${tmprepo}" --tree=dir="${rootfs_overrides}" -b cosa-overrides-rootfs \
+        ostree commit --repo="${tmprepo}" --tree=dir="${rootfs_overrides}" -b overlay/cosa-overrides-rootfs \
           --owner-uid 0 --owner-gid 0 --no-xattrs --no-bindings --parent=none \
           --timestamp "${git_timestamp}"
           cat >> "${override_manifest}" << EOF
 ostree-override-layers:
-  - cosa-overrides-rootfs
+  - overlay/cosa-overrides-rootfs
 EOF
     fi
 }