
Turns out all variants of start.elf and fixup.dat are needed (depending on what's in config.txt). I was under the mistaken impression that you were supposed to rename one of the variants to switch using them, but nope.
*** NixOS *** NixOS is a Linux distribution based on the purely functional package management system Nix. More information can be found at http://nixos.org/nixos and in the manual in doc/manual.