Provide XML for Drupal's builtin update status checks

If the CiviCRM site outputs a feed appropriate for use with https://www.drupal.org/node/542202#project%20status%20url then Drupal's built-in update mechanism will be able to report on necessary CiviCRM upgrades.

This would duplicate functionality already in CiviCRM, but would improve the experience of getting update notifications for most sites as it extends CiviCRM update notification to Drupal's email updates and other tools which tie into the Drupal update system.