ofono: fix missing systemd and dbus files

Ofono's service file was not being installed when systemd was selected
as the init system.

Without ofono.conf in /etc/dbus-1/system.d, ofonod cannot take its
dbus name on the system bus and will fail to start with a permission
denied error.

Signed-off-by: Mike Williams <mike@mikebwilliams.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>