systemd: Mitigate /var/tmp type mismatch issue

systemd: Mitigate /var/tmp type mismatch issue

The base-files recipe provides /var/tmp -> /var/volatile/tmp symlink
which is in conflict with systemd upstream tmpfiles.d/tmp.conf which
defines it as a directory (or subvolume on btrfs).

This generates following error in journal:

  Jul 03 15:37:21 qemux86-64 systemd-tmpfiles[158]: "/var/tmp" already exists and is not a directory.

Mitigate the issue by defining /var/tmp as symlink corresponding to
the one created by base-files.

Signed-off-by: Niko Mauno <niko.mauno@vaisala.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>