Introduce a tier-0 image

This is basically just:

- kernel
- systemd
- selinux-policy-targeted
- bootc

Notably it doesn't have `rpm-ostree` or `rpm`, or many other things.
It also doesn't even have `linux-firmware`.

And no `openssh`!

It's almost certain that you need to derive from this, but
it should be a suitable starting point.

TODO: Add something like

```
$ dnf-system-bootstrap
Installing packaging tools from quay.io/fedora/fedora-boot-dnf@sha256:abcd...)
 # This would be basically all the packages not in tier-0 that
 # are enough to give `dnf install`
$ dnf install cowsay
 # Install critical stuff
$ dnf system-bootstrap remove
 # Remove everything that we added for the package system, that isn't
 # a dependency of what the user wants!
```

(In theory we could make this work with multi-stage builds, but
 it's a little hard)

tier-0/kernel.yaml

@@ -0,0 +1,6 @@
+# Enable the Linux kernel; see also kernel-rt.
+packages:
+ - kernel
+
+exclude-packages:
+  - kernel-debug