systemd: add option to enable kdbus support

In the future when the kernel offically supports kdbus, this option
should automatically enable the kernel kdbus config option(s).

Signed-off-by: Ryan Barnett <ryanbarnett3@gmail.com>
Tested-by: Mike Williams <mike@mikebwilliams.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>