systemd: Revert inappropriate use of ${systemd_system_unitdir}

This is a partial revert of commit 5ace3ada (systemd:
'${systemd_unitdir}/system' => '${systemd_system_unitdir}') where
the string '${systemd_unitdir}/system' matched something other than the
intended directory.

Signed-off-by: Peter Kjellerstedt <peter.kjellerstedt@axis.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>