initscripts: Updates for populate-volatile.sh:

initscripts: Updates for populate-volatile.sh:
- Add support for "update" as a parameter to force regeneration of
  the cache when run (if the cache is not being used it's ignored).
- When generating the cache add *all* needed commands to the cache,
  not just those that were actually needed on this run. Without this
  only the additional directories created during an update will be
  added to the cache.
Anything that calls populate-volatile.sh during postinst should now
pass update as a parameter. Currently (and before this change) if the
cache is enabled then any call to populate-volatile.sh during
postinst does nothing, generally breaking anything that uses
volatiles that is installed after the first boot of the device.