mkdmg.sh script used by binarycreator doesn't actually generate random file name

The binarycreator uses a mkdmg.sh helper script at runtime to create .dmg file if requested. This script performs the following assignment:

DMG=`mktemp "/tmp/$VOL.XXXXXX.dmg"`

This doesn't actually create a random file name. OS X uses the BSD implementation of mktemp. The man page explains:

The trailing `Xs' are replaced with the current process number and/or a unique letter combination.

(emphasis by me).

Hence, on OS X, the `XXXXXX` part should be at the end of the template. Alas, the patch isn't that trivial since that causes the `hdiutil` used further down in the script to no longer recognize the image file format correctly.