Various changes and fixes with the general aim to support netbooting NixOS

by serving NixOS images from a NixOS machine to anything supporting PXE.

The described concept works alright, but the main piece of code is far from being ready for the general public (ugly hacks), but I got tired of rebasing and stashing these changes.
Yep. Most of these changes are irrelevant w.r.t. each other. They are related by being by-products of an effort to boot NixOS in a classroom with PXE.