Move site system settings from variable to config.
function system_update_8012() {
update_variables_to_config('system.site', array(
'site_name' => 'name',
'site_mail' => 'mail',
'site_slogan' => 'slogan',
'site_frontpage' => 'page.front',
'site_403' => 'page.403',
'site_404' => 'page.404',
'drupal_weight_select_max' => 'weight_select_max',
));
}