package/systemd: better patch to avoid ln --relative

We currently have two patches that address the ln --relative issue
differently. The first one changes the behaviour to generate absolute
symlinks, which is incorrect; the second provides an ad-hoc solution
for a single case.

Replace both of them with a single patch that mimics ln --relative when
the host ln does not support it.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>