systemd: don't build systemd-firstboot by default

systemd: don't build systemd-firstboot by default

systemd-firstboot is never invoked since systemd's first boot detection
logic checks whether /etc/machine-id exists. Since the file is created
automatically by systemd.mk, systemd will never detect first boot and
therefore the systemd-firstboot.service unit file will never get run.

Additionally, if /etc/machine-id is removed to allow systemd-firstboot
to run, it interactively prompts for the system locale. This makes it
seem unlikely that an embedded system would want to use it.

Signed-off-by: Frank Hunleth <fhunleth@troodon-software.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>