Properly implement metapackage-only builds

Rarely we want to just rebuild the metapackage and not do a full kernel build. https://gist.github.com/legoktm/82e08e796a977a5850fba3d13b03d6a5 (on the kernel-6 branch) allows for that. We should implement that as a `METAPACKAGE_ONLY=1` type variable.