minimal: Require kernel.yaml

We have a legacy of trying to support using e.g. kernel-rt. But
it adds complexity in the inheritance because minimal/manifest.yaml
isn't standalone, it also needs a kernel.

As part of custom base images I want to simplify this.

In order to use kernel-rt, we'll just say that you build a
minimal base, and then swap to kernel-rt as a secondary step
for now.

Signed-off-by: Colin Walters <walters@verbum.org>

fedora-minimal.yaml

@@ -5,4 +5,3 @@ metadata:
 include:
   - fedora-generic.yaml
   - minimal/manifest.yaml
-  - minimal/kernel.yaml