start modularising NixOS config for several hosts

First step in modularising the NixOS config, with a focus on separation of host-specific and common configs.
Common modules still need to be split up and refined, several TODOs and FIXUPs remain in code. But the config builds fine on thinknix.

Roughly based on/ inspired by https://johns.codes/blog/organizing-system-configs-with-nixos#using-nixos

common/nitrokey.nix

@@ -0,0 +1,11 @@
+{ pkgs, lib, ...}:
+{
+hardware.nitrokey.enable = true;
+environment.systemPackages = [ pkgs.pynitrokey ];
+nixpkgs.config.allowUnfreePredicate = pkg: builtins.elem (lib.getName pkg) [
+   "nrfutil"
+   "pc-ble-driver-py"
+   "pc-ble-driver"
+   "pypemicro"
+];
+}