Add platform-independent build

[lalten]

This PR makes the software buildable on Linux (probably any platform actually).
Changes are:

• to include dfuutils as submodule to build from source
  • dfuutil needed a small patch to be linux-compatible. I created PR Remove stdafx.h qba667/dfuutils#1 for this upstream. If that gets merged, the submodule should be updated to point at upstream again.
• small modification (make debug in main.c static) to be able to build dfuutils and the updater in the same project
• Addition of the Qt project file (NV14-updater.pro)
• libusb-1.0 is assumed to be available as system dep

To build this project one would now have to

git submodule update --init
qmake
make

Downloading, flashing, etc works :)