Small fixes

1. You broke make doxygen by including *.txt inside Demos/Device/LowLevel/BulkVendor/. That fails for me while processing the license text (Demos/Device/LowLevel/BulkVendor/WindowsDriver/license/libusb0/installer_license.txt) unsurprisingly. :)

2. I like clean repositories so a complete .gitignore is important to me. I am not sure if this change is correct and complete (it seems to handle make doxygen in the root dir fine though).