installer: Generate 'set-xorg-configuration' snippet.

* gnu/installer/services.scm (system-services->configuration): Add
'set-xorg-configuration' service.
* gnu/installer/steps.scm (format-configuration): Add 'xorg' service
module.

gnu/installer/services.scm

@@ -102,4 +102,13 @@ (define (system-services->configuration services)
                        '%base-services)))
     (if (null? snippets)
         `((services ,base))
-        `((services (cons* ,@snippets ,base))))))
+        `((services (cons* ,@snippets
+
+                           ,@(if desktop?
+                                 ;; XXX: Assume 'keyboard-layout' is in scope.
+                                 '((set-xorg-configuration
+                                    (xorg-configuration
+                                     (keyboard-layout keyboard-layout))))
+                                 '())
+
+                           ,base))))))

gnu/installer/steps.scm

@@ -215,7 +215,7 @@ (define (format-configuration steps results)
                   '())))
           steps))
         (modules '((use-modules (gnu))
-                   (use-service-modules desktop networking ssh))))
+                   (use-service-modules desktop networking ssh xorg))))
     `(,@modules
       ()
       (operating-system ,@configuration))))