prepare-root: Unify root.transient with composefs

prepare-root: Unify root.transient with composefs

First, I was totally wrong and composefs handles being passed
an upperdir itself, we don't need to stack overlayfs.

Next, there's really no reason to support `root.transient`
*without* a backing composefs.  The legacy ostree bind mount
and readonly `/usr` is just that - legacy.

Finally, we actually *must* do this to enable both composefs
and transient root, because the prepare-root flow assumes
that it just needs to `MS_MOVE` a *single* mount for the root,
not a stack.