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>
1c6bb7f917