Fix machine-id mechanism for Buster.

The logic wrt /etc/machine-id changed between Buster and Bullseye.
While on Bullseye the file should not exist, on Buster the file must
exist, but be empty, in order to generate a new machine-id on first
boot.
It seems that /var/lib/dbus/machine-id is a symlink to /etc/machine-id
on Buster, while a separate file on Bullseye, so nothing needs to be
done with that file/symlink.

Signed-off-by: Diederik de Haas <didi.debian@cknow.org>

raspi_master.yaml

@@ -161,6 +161,9 @@ steps:
   #   populated during installation.
   #
   # Note this will also trigger ConditionFirstBoot=yes for systemd.
+  # On Buster, /etc/machine-id should be an emtpy file, not an absent file
+  # On Bullseye, /etc/machine-id should not exist in an image
   - chroot: /
     shell: |
       rm -f /etc/machine-id /var/lib/dbus/machine-id
+      touch /etc/machine-id