systemd.bbclass: Dont use libdir and base_libdir for units
systemd always uses /lib and /usr/lib to store unit files
so using libdir and base_libdir is incorrect. It will work
where libdir is usr/lib and base_libdir is /lib but wont work
when say its /lib64
Add a check to make sure that SYSTEMD_PACKAGES are part of PACKAGES
too, otherwise error out
