nixos: loader: added generic config.boot.loader.timeout option

timeout options of grub and gummiboot will inherit the value of this
option by default.

nixos/modules/system/boot/loader/loader.nix

@@ -0,0 +1,15 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+{
+    options = {
+        boot.loader.timeout =  mkOption {
+            default = 5;
+            type = types.nullOr types.int;
+            description = ''
+              Timeout (in seconds) until loader boots the default menu item. Use null if the loader menu should be displayed indefinitely.
+            '';
+        };
+    };
+}