This is a follow-up to #401. Fedora CoreOS now ships with an archive repo which allows reliable package layering operations. However, there is still work remaining to make it easier to use. Right now, users have to create a systemd unit which runs rpm-ostree install --reboot ... which is not great.

Specifically:

1. we need a better interface for package layering via Ignition, whether that be via file drop-ins translated by FCOS glue code, or drop-ins read by rpm-ostree directly (related: rethinking origins rpm-ostree#2326)
2. we want "Ignition-like" semantics for the extensions: this means that the extensions ideally should be present from the get go in the real root on the first boot (or we should fail the boot if something goes wrong)
3. we need an FCC-friendly way to specify these packages, either directly having users use the file drop-in interface or via sugar

The sugar that was proposed in #401 was something like:

extensions:
- usbguard