Here are some patches to create a 'kernel-updates' Cuirass
specification, so that CI will build kernel-related things based on
changes to a kernel-updates Git branch.

The first patch creates a manifest of packages named "linux-libre-*" for
the Guix source tree, and the second patch creates the Cuirass spec for
maintenance.git.

I can test the manifest with `guix weather`, but I'm not sure how to
test the Cuirass spec.

Advice is welcome!