add an init config for RBPi 4

In the initial image flakes are not enabled. This solves this problem.

systems/raspi4/init_config.nix

@@ -0,0 +1,40 @@
+{ config, pkgs, lib, ... }:
+{
+  imports = [ "${fetchTarball "https://github.com/NixOS/nixos-hardware/archive/32f61571b486efc987baca553fb35df22532ba63.tar.gz" }/raspberry-pi/4" ];
+
+  fileSystems = {
+    "/" = {
+      device = "/dev/disk/by-label/NIXOS_SD";
+      fsType = "ext4";
+      options = [ "noatime" ];
+    };
+  };
+
+  hardware.raspberry-pi."4".fkms-3d.enable = true;
+  hardware.raspberry-pi."4".audio.enable = true;
+  hardware.pulseaudio.enable = true;
+
+  environment.systemPackages = with pkgs; [
+    raspberrypi-eeprom
+  ];
+
+  services.openssh.enable = true;
+  networking.hostName = "nixos";
+
+  nix = {
+    package = pkgs.nixFlakes;
+    extraOptions = ''
+      experimental-features = nix-command flakes
+      warn-dirty = false
+    '';
+  };
+
+  users = {
+    mutableUsers = false;
+    users."nixos" = {
+      isNormalUser = true;
+      initalPassword = "password";
+      extraGroups = [ "wheel" ];
+    };
+  };
+}