Enable composefs by default

We really want to aim to eventually enable this by default, let's
test things out in rawhide.

A thing that this is known to break is the "chattr -i" hack
for new toplevel dirs (xref coreos/rpm-ostree#337 )

Basically if you want that, you either need to make a derived image,
or enable transient root.

manifest.yaml

@@ -9,6 +9,9 @@ repos:
 
 include: manifests/fedora-coreos.yaml
 
+ostree-layers:
+  - overlay/08ostree-prepare-root
+
 # Ship this in rawhide because we want to enable it in the future, also
 # to shake out any integration issues.
 packages:

overlay.d/08ostree-prepare-root/usr/lib/ostree/prepare-root.conf

@@ -0,0 +1,3 @@
+[composefs]
+enabled = true
+