main: introduce /etc/machine-id
This is supposed to play the same roles /var/lib/dbus/machine-id, however fixes a couple of problems: - It is available during early boot since it is stored in /etc - Removes the ID from the D-Bus context and moves it into a system context, thus hopefully lowering hesitation by people to use it. - It is generated at installation time. If the file is empty at boot time it will be mounted over with a randomly generated ID, which is not saved to disk. This is useful to support state-less machines with no transient or writable /etc configuration.