Facedancer backend

[xairy]

It should be possible to use Raw Gadget as a backend for Facedancer. At least for UDCs with configurable (direction and transfer type) gadget endpoints.

[trevor403]

This would be quite interesting, it would be great to have accessible tools for teaching USB, like the PI Zero or even the some Intel Atom and Quark boards with UDC capabilities.

[xairy]

kirill9617 implemented a prototype Facedancer backend based on Raw Gadget.

Some notes:

1. Facedancer processes all requests in a single service_irqs() handler, which is incompatible with Raw Gadget's blocking fetch/read/write ioctls. A potential solution is to add timeouts for the ioctls.

2. Running some of Facedancer examples fails, the failures looks similar to how to send configuration descriptor in 2 parts #29.

[xairy]

Finished up the prototype and made most Facedancer examples work with Dummy and dwc2 UDCs, see the details here.