overlayfs: add systemd unit path prefix to FILES:${PN} array

overlayfs: add systemd unit path prefix to FILES:${PN} array

The 'FILES:${PN}' array is missing the systemd unit dir prefix causing
them to not be packaged and the build fails with the
`installed-vs-shipped` error.

This adds the `systemd_system_unitdir` variable in front of every unit
file added with this class.

Signed-off-by: Claudius Heine <ch@denx.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>