[WIP] Cleanup base setup

[robin-reckmann]

base_setup.sh is not well maintained. This is especially an issue, as it is the first script in the docker build and any changes here forces a nearly complete rebuild.

With this PR, base_setup.sh will be cleaned up, so that it creates a bootable minimal Ubuntu with some connectivity, but not more. After this cleanup there shouldn't be any need to touch this file, except for major changes (e.g. mainline kernel).