backend: add Raw Gadget backend

This is the latest version of the Raw Gadget backend implementation by
@akvadrako sent in greatscottgadgets#160 (with a few minor changes dropped or separated
into other patches).

The patch that follows contains fixes for that implementation.

Co-developed-by: Andrey Konovalov <andreyknvl@gmail.com>

Author: akvadrako

README.md

@@ -100,6 +100,7 @@ export BACKEND=greatfet
  * All GoodFET-based Facedancers, including the common Facedancer21 (```BACKEND=goodfet```)
  * RPi + Max3241 Raspdancer boards (```BACKEND=raspdancer```)
  * HydraDancer and HydraUSB3 boards (```BACKEND=hydradancer```)
+ * Any Linux board with [Raw Gadget](https://github.com/xairy/raw-gadget) support (```BACKEND=rawgadget```)
 
 Note that hardware restrictions prevent the MAX3420/MAX3421 boards from emulating
 more complex devices -- there's limitation on the number/type of endpoints that can be
@@ -114,7 +115,6 @@ Note actual FaceDancer 3.0 does not work on Windows(some issues in pyusb...) and
 
 ## What boards could be supported soon?
 
- * Any Linux computer with gadgetfs support (e.g. the Pi Zero or Beaglebone Black)
  * Anything supporting USB-over-IP.
 
 ## What features do you plan on adding?

facedancer/backends/__init__.py

@@ -6,5 +6,6 @@
     "greathost",
     "libusbhost",
     "moondancer",
-    "hydradancer"
+    "hydradancer",
+    "rawgadget",
 ]