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
147641fc3e