dbus: add message-bus PACKAGECONFIG

dbus: add message-bus PACKAGECONFIG

There's no need to build an actual message bus in native or nativesdk
environments, as if DBus is needed in those environments then it will be
used to connect to the existing session or system bus.

Add a PACKAGECONFIG for the message bus, and wrap the packaging logic
with a PACKAGECONFIG check.

Expat is only needed by the bus, so this reduces the mandatory build
dependencies in the native case.

This means we can merge the overridden do_install appends, as native and
nativesdk don't have the message bus enabled so they don't install
dbus-launch.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Mathieu Dubois-Briand <mathieu.dubois-briand@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>