Form builder submit handler; Handle submission for RSS feeds settings.
function system_rss_feeds_settings_submit($form, &$form_state) {
config('system.rss')
->set('channel.description', $form_state['values']['feed_description'])
->set('items.limit', $form_state['values']['feed_default_items'])
->set('items.view_mode', $form_state['values']['feed_item_length'])
->save();
}