dbus: use the systemd class to handle the unit files

dbus: use the systemd class to handle the unit files

Originally, the dbus recipe couldn't use the systemd class because there
was a circular dependency between systemd and dbus.

However, since systemd v209 in 2014[1] systemd hasn't needed libdbus, as
it has it's own implementation of the client library. DBus does not use
the systemd libraries, so there is no circular dependency.

The dbus build was already was installing the service and socket files,
so we are installing them again. Remove the installation of the units.

Manually mask dbus-1.service by simply creating a symlink, instead of
depending on systemctl and using an postinstall script.

Signed-off-by: Ross Burton <ross.burton@arm.com>
[1] dbus: 718db96199 ("core: convert PID 1 to libsystemd-bus")

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