base-systemd: fix /var, /run, resolv.conf setup

In order for systemd to run properly, the /var and /run directories of
the target should be changed:

- /run will be mounted and populated by systemd, thanks to the
  /usr/lib/tmpfiles.d/*.conf files.
- /var/run and /var/lock should be symlinks to /run
- /var/tmp *should not* be a symlink to /tmp, as it will be mounted and
  populated by systemd.

Also set /etc/resolv.conf as a symlink to
/run/systemd/network/resolv.conf if systemd-networkd is used.

Signed-off-by: Eric Le Bihan <eric.le.bihan.dev@free.fr>