systemd: revert use of ln relative option.

Systemd build system now uses the `--relative` option from `ln(1)`.
This option was added to GNU coreutils 8.16, which is not widely
deployed yet by GNU/Linux distributions (not available in Debian Wheezy
for example).

Fixes: http://autobuild.buildroot.net/results/354/3546c003a8fcbb36ef5ba29c00a96e473b927ecb/



Signed-off-by: Eric Le Bihan <eric.le.bihan.dev@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>