Add a condition controlled by admin to avoid boost cache flush on every update.php

I noticed that update.php call drupal_flush_all_caches function which call boost_flush_caches(). As there is frequent update of drupal modules, update.php is launched frequently. Then some high expiration dates are never reached. The boost cache is flushed before.

It would be interesting too add a condition to avoid boost cache flush if a update.php is executed.

The condition would be controlled by a setting in the admin interface.