Set machine-id to uninitialized to trigger first boot

According to the systemd documentation[1], if /etc/machine-id is empty
it will be populated with a unique value, but not in a way which
triggers an actual first boot event (running units with
ConditionFirstBoot=yes set)

This change writes "uninitialized" to /etc/machine-id to ensure that
systemd-firstboot.service actually runs, and other units can use
first-boot-complete.target as a dependency to trigger on first boot.

Since /var/lib/dbus/machine-id is sometimes a symlink to
/etc/machine-id, it is truncated before writing to /etc/machine-id.

On older versions of systemd before first boot semantics were
formalised, any non-uuid value will trigger a new machine-id to be
generated, so "uninitialized" also works.

[1] https://www.freedesktop.org/software/systemd/man/machine-id.html#First%20Boot%20Semantics

Change-Id: I77c35e51a3da2e8a6b5a2c80d033a159b303c9af

diskimage_builder/elements/sysprep/finalise.d/99-clear-machine-id

@@ -6,10 +6,17 @@ fi
 set -eu
 set -o pipefail
 
-if [ -e /etc/machine-id ]; then
-    > /etc/machine-id
-fi
-
 if [ -e /var/lib/dbus/machine-id ]; then
     > /var/lib/dbus/machine-id
 fi
+
+# According to documented First Boot Semantics writing "uninitialized" will trigger the full
+# first boot behaviour.
+# https://www.freedesktop.org/software/systemd/man/machine-id.html#First%20Boot%20Semantics
+# On older versions of systemd before first boot semantics were formalised, any non-uuid value
+# will trigger a new machine-id to be generated, so "uninitialized" also works.
+# Previously this was done here by truncating /etc/machine-id.
+if [ -e /etc/machine-id ]; then
+    echo uninitialized > /etc/machine-id
+fi
+