fix to get the build script working under a minimal lxc environment

First of all, many thanks for your great work!

I have created a minimal lxc container with ubuntu precise so that I can use your cubietruck build script.
The first commit ensures that the 'partprobe' program is present (through package parted).
The second commit enables the arm binformat in case it is not yet enabled.
The third commit eliminates one loopback file.
I had some Issues with loopback devices on lxc, this was an attempt to find out why my problem exists :)
The problem is actually still here but I think the change should be valid anyway.

Regards,
-phil