buildroot

package: Fix source path for systemd's service

Otherwise the link would appear broken in output/target it's not that
big a deal but at least it's coherent with what we already have.

Signed-off-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>