bootserver: update DNS records required by airprint after ddns init
bootserver: update airprint DNS records once per 15mins

This ensures printer permission/visibility changes in Puavo come into
effect with max 15min delay.

This kind of recurring and stupid update-loop is required until
bootservers can subscribe to Puavo event queues.
bootserver: install airprint DDNS update cron script
bootserver: fix trivial typo in airprint update cron script
Use pdip-image.excludes in image creation to exclude
some directories.