Restructure flake.nix for a simpler config for building different images off the same NixOS config.

nix/configuration/roles/image_based_appliance/default.nix

@@ -0,0 +1,29 @@
+{
+  config,
+  lib,
+  ...
+}:
+
+{
+  imports = [ ];
+
+  options.me = {
+    image_based_appliance.enable = lib.mkOption {
+      type = lib.types.bool;
+      default = false;
+      example = true;
+      description = "Whether we want to install image_based_appliance.";
+    };
+  };
+
+  config = lib.mkIf config.me.image_based_appliance.enable (
+    lib.mkMerge [
+      {
+        # Do not install nix. A full new image must be built to update
+        # the machine.
+        nix.enable = false;
+        system.switch.enable = false;
+      }
+    ]
+  );
+}