Add stateVersion option

modules/build/initial-build/nix-on-droid.nix.default

@@ -24,6 +24,9 @@
     #unzip
   ];
 
+  # Read the changelog before changing this value
+  system.stateVersion = "19.09";
+
   # After installing home-manager channel like
   #   nix-channel --add https://github.com/rycee/home-manager/archive/master.tar.gz home-manager
   #   nix-channel --update

modules/module-list.nix

@@ -11,5 +11,6 @@
   ./environment/path.nix
   ./home-manager.nix
   ./user.nix
+  ./version.nix
   <nixpkgs/nixos/modules/misc/assertions.nix>
 ]

modules/version.nix

@@ -0,0 +1,33 @@
+# Licensed under GNU Lesser General Public License v3 or later, see COPYING.
+# Copyright (c) 2019 Alexander Sosedkin and other contributors, see AUTHORS.
+
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+{
+
+  ###### interface
+
+  options = {
+
+    system.stateVersion = mkOption {
+      type = types.enum [ "19.09" ];
+      default = "19.09";
+      description = ''
+        It is occasionally necessary for nix-on-droid to change
+        configuration defaults in a way that is incompatible with
+        stateful data. This could, for example, include switching the
+        default data format or location of a file.
+        </para><para>
+        The <emphasis>state version</emphasis> indicates which default
+        settings are in effect and will therefore help avoid breaking
+        program configurations. Switching to a higher state version
+        typically requires performing some manual steps, such as data
+        conversion or moving files.
+      '';
+    };
+
+  };
+
+}