nixos: add AppStream module

nixos/modules/config/appstream.nix

@@ -0,0 +1,25 @@
+{ config, lib, ... }:
+
+with lib;
+{
+  options = {
+    appstream.enable = mkOption {
+      type = types.bool;
+      default = true;
+      description = ''
+        Whether to install files to support the 
+        <link xlink:href="https://www.freedesktop.org/software/appstream/docs/index.html">AppStream metadata specification</link>.
+      '';
+    };
+  };
+
+  config = mkIf config.appstream.enable {
+    environment.pathsToLink = [ 
+      # per component metadata
+      "/share/metainfo" 
+      # legacy path for above
+      "/share/appdata" 
+    ];
+  };
+
+}

nixos/modules/module-list.nix

@@ -11,6 +11,7 @@
   ./config/xdg/icons.nix
   ./config/xdg/menus.nix
   ./config/xdg/mime.nix
+  ./config/appstream.nix
   ./config/gtk/gtk-icon-cache.nix
   ./config/gnu.nix
   ./config/i18n.nix