nixos: move nixosLabel to its own module

nixos/modules/misc/label.nix

@@ -0,0 +1,32 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+let
+  cfg = config.system;
+in
+
+{
+
+  options.system = {
+
+    nixosLabel = mkOption {
+      type = types.str;
+      description = ''
+        NixOS version name to be used in the names of generated
+        outputs and boot labels.
+
+        If you ever wanted to influence the labels in your GRUB menu,
+        this is the option for you.
+      '';
+    };
+
+  };
+
+  config = {
+    # This is set here rather than up there so that changing it would
+    # not rebuild the manual
+    system.nixosLabel = mkDefault cfg.nixosVersion;
+  };
+
+}

nixos/modules/misc/version.nix

@@ -32,14 +32,6 @@ in
       '';
     };
 
-    nixosLabel = mkOption {
-      type = types.str;
-      description = ''
-        Label to be used in the names of generated outputs and boot
-        labels.
-      '';
-    };
-
     nixosVersion = mkOption {
       internal = true;
       type = types.str;
@@ -89,7 +81,6 @@ in
     system = {
       # These defaults are set here rather than up there so that
       # changing them would not rebuild the manual
-      nixosLabel   = mkDefault cfg.nixosVersion;
       nixosVersion = mkDefault (cfg.nixosRelease + cfg.nixosVersionSuffix);
       nixosRevision      = mkIf (pathIsDirectory gitRepo) (mkDefault            gitCommitId);
       nixosVersionSuffix = mkIf (pathIsDirectory gitRepo) (mkDefault (".git." + gitCommitId));

nixos/modules/module-list.nix

@@ -60,6 +60,7 @@
   ./misc/extra-arguments.nix
   ./misc/ids.nix
   ./misc/lib.nix
+  ./misc/label.nix
   ./misc/locate.nix
   ./misc/meta.nix
   ./misc/nixpkgs.nix