prepare-root: Introduce `ostree/prepare-root.conf`

prepare-root: Introduce `ostree/prepare-root.conf`

Using the repository configuration for configuration of this
program was always a bit hacky.

But actually with composefs, we really must validate
the target root *before* we parse anything in it.

Let's add a config file for `ostree-prepare-root` that can live
in the initramfs, which will already have been verified.

In the future we'll also add configuration for composefs here.

We expect OS builders to drop this in `/usr/lib/ostree/prepare-root.conf`,
but system local configuration can live in `/etc`.