systemd.bbclass: automatically extend RDEPENDS for *-systemd packages

* append systemd functionalty moved into systemd_add_rdepends
* append base-pgk in case receipe does not set RDEPENDS for *-systemd package

Signed-off-by: Andreas Müller <schnitzeltony@googlemail.com>
Signed-off-by: Koen Kooi <koen@dominion.thruhere.net>