witherspoon: Use systemd_unitdir

Somewhere between systemd 234 and 237 the install process stopped making
/usr/lib/systemd/network for us.  When we move to Yocto 2.5 this will
result in an install failure.

bitbake.conf provides a variable for /lib/systemd.  Use it, as it's the
correct location for these configuration/unit files anyhow.

Tested: Booted QEMU witherspoon image to shell:
Change-Id: I000b6f1b5548efb920d31226558159cba3deb8d6
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
1 file changed