Allows 'update modules' to be added to a Drupal installation, instead of or in addition to using hook_update_N(.
Each module should add:
update type=before or update type=after
to the .info file.
When drush updb runs, all modules that are found will be installed, either before or after the actual hook_update_N() runs.
This is primarily useful for sites under heavy development to avoid hook_update_N() conflicts.