Force mktemp to honor $TMPDIR in merge-squashfs script

[schrockwell]

This is a proposed solution to nerves-project/nerves#876.

On macOS, mktemp will create a temporary directory in /var/... which is on a case-insensitive volume. The merge-squashfs script will fail when Buildroot packages contain files that differ only in their case.

By adding the -p '' argument to mktemp, it is forced to honor the $TMPDIR environment variable, which the user could specify ahead of time as a special directory on a case-sensitive volume.

[fhunleth]

Adding -p '' seems reasonable regardless. It took me a bit to see if this change has any portability issues, but I really think it's safe.

Regarding using $TMPDIR as a workaround for the case-insensitivity issue, I think this is the most practical near-term solution. Getting rid of the need for merge-squashfs is my favorite one, but that's definitely more work.

Thank you for the PR.

[jjcarstens]

Thanks!