Include optional CUPS drivers

We are missing a few optional CUPS drivers in the current images. I found the following ones:

• c2esp
• dymo-cups-drivers
• pnm2ppa
• ptouch-driver
• splix

Given that they are really small packages, and not obvious for users to find out and overlay, I think we should include those directly in the image, maybe via the comps group.

We are also not including the foomatic and foomatic-db-ppds packages but I don't know exactly what their purpose is or if that would help with supporting more printers.