package/systemd: add dependency on host-coreutils

package/systemd: add dependency on host-coreutils

This is needed as systemd has gained a dependency on realpath(1) which
was introduced in coreutils too recently for our supported distro to
have it (Ubuntu 14.04 does not have it from coreutils, although there is
a dedicated package for it).

This also means that we now have a ln that understands --relative, so we
can drop our workaround, that upstream said they would never accept
anyway [0].

[0] https://github.com/systemd/systemd/pull/5682

Fixes:
    http://autobuild.buildroot.org/results/a9a/a9a285e482285d062892bab0d1a2e2f89928c92d/
    http://autobuild.buildroot.org/results/6f5/6f5b1065859d866af6fa719f611c3ea7f4b88760/
    ...

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