Convert update's system_config_form() to SystemConfigFormBase

Updated the patch as an example of how things have to look like after #1934832: Provide a dedicated approach for using forms in routes got committed. What the interdiff is missing is that we no longer need to register a service for the new form class.

Add sprintweekend tag.

+++ b/core/modules/update/update.module
@@ -170,8 +170,7 @@ function update_menu() {
-    'page callback' => 'drupal_get_form',
-    'page arguments' => array('update_settings'),
+    'page callback' => 'NOT_USED',
     'access arguments' => array('administer site configuration'),

This should now be a route key, specifying the route name.

Other than that, this looks fine.

+++ b/core/modules/update/lib/Drupal/update/Form/UpdateFormBase.php
@@ -0,0 +1,46 @@
+/**
+ * Provides a generic locale form class.
+ */
+abstract class UpdateFormBase extends SystemConfigFormBase implements ControllerInterface {

Uh, whut? Why are you defining a class that duplicates SystemConfigFormBase and then not using it...?

+++ b/core/modules/update/lib/Drupal/update/UpdateSettingsForm.php
@@ -0,0 +1,124 @@
+  public function validateForm(array &$form, array &$form_state) {
+    parent::validateForm($form, $form_state);

Seems a bit weird that in all the other conversions we had the calls to parent methods at the end of the methods and here is at the top, can we move this one as well?

Looking good now :)

Committed and pushed to 8.x. Thanks!