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/module-list.nix

···
   ./system/boot/kernel.nix
   ./system/boot/kexec.nix
   ./system/boot/loader/efi.nix
+  ./system/boot/loader/loader.nix
   ./system/boot/loader/generations-dir/generations-dir.nix
   ./system/boot/loader/grub/grub.nix
   ./system/boot/loader/grub/ipxe.nix

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

···
       };
 
       timeout = mkOption {
-        default = 5;
+        default = if (config.boot.loader.timeout != null) then config.boot.loader.timeout else -1;
         type = types.int;
         description = ''
           Timeout (in seconds) until GRUB boots the default menu item.

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

···
     };
 
     timeout = mkOption {
-      default = null;
+      default = if (config.boot.loader.timeout != null) then
+        (if (config.boot.loader.timeout == 0) then null else config.boot.loader.timeout)
+        else config.boot.loader.timeout;
 
       example = 4;
 

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

···
+{ 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.
+            '';
+        };
+    };
+}