If a recipe inherits systemd, installs a systemd unit into ${systemd_unitdir} and doesn't explicitly mention it in FILES and you aren't using systemd (i.e. you don't have systemd in DISTRO_FEATURES) you'll get an unpackaged file warning during do_package, because the code in systemd.bbclass that normally adds ${systemd_unitdir} to FILES is explicitly not called. This issue can be seen in various recipes in meta-networking e.g. ypbind-mt. I suspect these unit files ought to just be deleted at the end of do_install by systemd.bbclass if systemd is not in DISTRO_FEATURES.
This seems like better solution http://lists.linuxtogo.org/pipermail/openembedded-core/2013-February/036169.html Remove duplicated do_installs and install systemd files by shared chunk of code in systemd.bbclass (like it was in meta-systemd) only when systemd is in DISTRO_FEATURES and remove them if they are installed by upstream and systemd isn't in DISTRO_FEATURES
Fix merged to master: http://git.yoctoproject.org/cgit/cgit.cgi/poky/commit/?id=67864ca79da08df752487a3a4e1a975546da123d