hostnamed: introduce new "embedded" chassis type

We really don't want to get lost in adding fridge, car, plane, drone, or
whatever else, hence add a generic term "embedded" cover all the cases
where the computer is just part of something bigger, and not at the
focus of things.
systemctl: when invokes as "reboot -f", sync()

We do this in the clean shutdown path in shutdown.c, hence we should do
is for "reboot -f", too.
README: mention that engineering services for systemd are available from endocode