Introduce a base package for version sync

The image and headers packages, if installed together, need to come from the same version. If they are not from the same version, things like dkms break down, as it can't build modules for the correct kernel. Right now this is enforced by a strict dependency from headers to image.

To allow independent installation of image and headers, we introduce an mostly empty base package (similar to gcc-x-base). Both image and headers packages gain a strict dependency on this new base package. This way the versions needs to match.