bitbake.conf: Change systemd_unitdir definition

bitbake.conf: Change systemd_unitdir definition

systemd_unitdir indicates the arch independent
files which are basically scripts and unit files
and systemd wants then to be in /lib always even
when base_libdir is  /lib64, hence we have to reflect
that and not use base_libdir to define it. Otherwise
on architectures where base_libdir is lib64 e.g. ppc64
or multilibbed x86_64 this wont work

(From OE-Core rev: 50e713d4ae35f9b5f5f2a515a95d77573610d707)

Signed-off-by: Khem Raj <raj.khem@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>