Currently, a loss of power after the machine-id was written but before
all units with ConditionFirstBoot=yes ran would lead to the next boot
finding a valid machine-id, thus not being marked first boot and not
re-running these units.
To make the first boot mechanism more robust, instead of writing
/etc/machine-id very early, fill it with a marker value "uninitialized"
and overmount it with a transiently provisioned machine-id. Then, after
the first boots completes (when systemd-machine-id-commit.service runs),
write the real machine-id to disk.
This mechanism is of course only invoked on first boot. If a first boot
is not detected, the machine-id is handled as previously.
Fixes: #4511
3023f2fead