Use specific cron file in /etc/cron.d

[cornelinux]

Backups, updates and cleanups as cronjobs are written to the file /etc/crontab.

We could write these entries to a new own file like /etc/cron.d/pi-appliance, which might keep things more in one place.
But in this case we would have to take care about migration, which might get a bit tricky to do this in a robust way.

Maybe in the postinst script, we could copy /etc/crontab to /etc/cron.d/pi-appliance if it does not exist, yet and remove the non-pi-lines.