mkimage: use /var/tmp by default instead of /tmp

Additionally, this can be overridden by setting the TMPDIR variable,
like this was already the case for the generic `mkimage.sh` script.

As explained in #6456, the rationale to use `/var/tmp` instead of `/tmp`
is that `/tmp` is often a small tmpfs filesystem with more restricted
rights.

Docker-DCO-1.1-Signed-off-by: Vincent Bernat <vincent@bernat.im> (github: vincentbernat)