machine-id and first boot notes

if /etc/machine-id does not exist and we're not still in initramfs then
systemd will enter "first boot" mode.
in this case /etc may not be mounted read-only or boot will fail.
first boot mode is not triggered if /etc/machine-id exists but is empty/invalid.

new in v247: if /etc/machine-id exists but contains the string "uninitialized"
then it is treated as if it were missing and first boot mode is entered.


--- obtaining machine id ---

systemd checks the following, in decreasing order of priority:
	systemd cmdline option '--machine-id'
	kernel parameter 'systemd.machine_id'
	file '/etc/machine-id'
	file '/var/lib/dbus/machine-id'  (unless this is a symlink)
	env variable 'container_uuid' if running in a container
	file '/sys/class/dmi/id/product_uuid' if running in a KVM container
	file '/sys/firmware/devicetree/base/vm,uuid' if running in a KVM container
until it finds a _valid_ uuid.
if none is found, a new random uuid is generated.


--- writing machine id ---

systemd will try to save the machine id to /etc/machine-id, unless it was
obtained from there.  if this fails then systemd will save it to /run/machine-id
instead and it then bind-mounts this on top of /etc/machine-id.  this is called
a "transient machine id" since it was not saved to persistent storage.

new in v247: in first boot mode the string "uninitialized" will be written to
/etc/machine-id and a transient machine id will be used until boot is complete
at which point systemd-machine-id-commit.service will make the machine id
presistent.  this ensures that if the first boot does not complete (e.g. power failure)
then first boot mode will be entered again on the next boot.


---- first boot mode ---

/run/systemd/first-boot will be created (as empty file).
it will be unlinked again when the (first) boot is finished.

ConditionFirstBoot for unit files will evaluate to true.
new in v246: this can be overridden using kernel parameter 'systemd.condition-first-boot'.

systemd will perform the equivalent of:
	systemctl --system --preset-mode=enable-only preset-all

When seeding the kernel entropy pool from the seed file '/usr/lib/systemd/random-seed', the
'user.random-seed-creditable' xattr on the seed file will be ignored hence the entropy pool
will only be credited if SYSTEMD_RANDOM_SEED_CREDIT=forced.