Add configuration for buildkit, in order to not have to use flags

[afbjorklund]

We can pass the setup as configuration, rather than flags.

That makes it easier to start from systemd units, and so on.

We want to use the containerd worker, by default.

-oci-worker false --containerd-worker true

And store the result in the "k8s.io" namespace.

--containerd-worker-namespace k8s.io

Helps with #9947

[afbjorklund]

/etc/buildkit/buildkitd.toml

https://github.com/moby/buildkit/blob/master/docs/buildkitd.toml.md

[worker.oci]
  enabled = false
[worker.containerd]
  enabled = true
  namespace = "k8s.io"

Need to do this for both ISO and KIC installations of buildkit