systemd: explicitly disable dbus

systemd: explicitly disable dbus

This is actually only used in a single test that we don't build, so
explicitly disable DBus instead of offering a PACKAGECONFIG that doesn't
do what you might expect (enable some systemd/dbus integration).

This doesn't change behaviour as this option wasn't enabled by default.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>