ospack: Empty machine-id so it's made unique on first boot

The apertis-customization package used to ship in tmpfiles.d/apertis.conf a
link from /etc/machine-id to /var/lib/dbus/machine-id to ensure that the
machine-id is unique.

This is now done automatically provided that /etc/machine-id is a empty file.

Signed-off-by: Emanuele Aina <emanuele.aina@collabora.com>