Enable composefs root

This partially reverts commit 7977ead6e4 and
effectively migrates the change from https://github.com/CentOS/centos-bootc-dev/pull/27/commits/8f5be0937144b602d8c1fdcaff4c51777b8cb254
and https://github.com/CentOS/centos-bootc-dev/pull/32/commits/14ab1c5847afbd5a47c5fd023c0720e4a6d99b31
to here.

Signed-off-by: Colin Walters <walters@verbum.org>

tier-0/ostree.yaml

@@ -7,9 +7,8 @@ postprocess:
     #!/usr/bin/env bash
     mkdir -p /usr/lib/ostree
     cat > /usr/lib/ostree/prepare-root.conf << EOF
-    # TODO disabled due to https://github.com/osbuild/bootc-image-builder/issues/149
-    #[root]
-    #transient = true
+    [composefs]
+    enabled = true
     [sysroot]
     readonly = true
     EOF