Form submission handler for system_site_information_settings().
function system_site_information_settings_submit($form, &$form_state) {
config('system.site')
->set('name', $form_state['values']['site_name'])
->set('mail', $form_state['values']['site_mail'])
->set('slogan', $form_state['values']['site_slogan'])
->set('page.front', $form_state['values']['site_frontpage'])
->set('page.403', $form_state['values']['site_403'])
->set('page.404', $form_state['values']['site_404'])
->save();
}