Raspberrypi fixes for OpenNi2

Dear OpenNi2-Maintainer(s).
I had given the OpenNi a try for RaspberryPi. 
After changing the stuff as seen in this Pull request i was able to run the non-Gui examples with an Xtion.

Could you please integrate it back into your repo, so others could benefit of it?

Things to note:
-> I defined a new Arch Armv6l, which is what uname -m prints. However, its up to you to decide how to call the arch on which the RPi runs.
-> There was a unused but uncompilable USB file which was enabled via an ifdef ARM. I commented it completely with an #if 0. So maybe your #ifdef ARM is not specific enough here (maybe it means android?).


Regards

Hauke