systemd: '${systemd_unitdir}/system' => '${systemd_system_unitdir}'

Repo-wide replacement to use newer variable to represent systemd
system unitdir directory.

Signed-off-by: Robert P. J. Day <rpjday@crashcourse.ca>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>