Arbitrary top level mount points and composefs

As we are looking at enabling composefs by default in Fedora CoreOS, the question of arbitrary top level mount points came again, especially for the podman machine use case (to be clarified again on the exact usage).

In the general case with bootc, anyone that want to use a top-level dir as mount point should be able to create this directory in a derived image.

This is however not possible if the name of those mount points is not known in advance / at boot time.

See:

• https://github.com/coreos/fedora-coreos-tracker/issues/1718
• https://fedoraproject.org/wiki/Changes/ComposefsAtomicCoreOSIoT
• https://github.com/coreos/rpm-ostree/issues/337

Another option is to use root.transient from ostree but that's a bigger change: https://github.com/ostreedev/ostree/pull/3114