Motivation for this change

This will make easier to set the font of the virtual console
in NixOS. Instead of specifing the full filepath of the psf
on can simply do:

console.font = "ter-powerline-v20b";
console.packages = [ pkgs.powerline-fonts ];

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested fonts in virtual console, X11 and GTK applications
• Tested execution of all binary files (none)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.