systemd.bbclass: run code in populate_packages_prepend only if ${D} exists

All further manipulations need to be performed only once. This modication
avoids unwanted behaviour when inheriting rm_work.bbclass [1]

[1] http://lists.linuxtogo.org/pipermail/openembedded-devel/2012-February/038067.html



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