moves libusb from vendored to submodule

This prevents them from being re-added in the future whenever someone
updates libusb. Also removes the need to manually remove those files.

Now has support for isochronous transfers in the WinUSB backend,
which may or may not work better than the UsbDk backend.

Check UsbDK availability at run time and activate UsbDk backend when
possible.

Fall back to current behavior when UsbDK not installed.

Signed-off-by: Dmitry Fleytman <dfleytma@...>
Signed-off-by: Sameeh Jubran <sameeh@...>

The usbdk backend is the only libusb backend that has official support
for isochronous transfers (which are required for Wii Speak,
microphones and cameras). And it's actively developed and maintained.

1.0.21 adds a usbdk backend, which may be helpful as it could be
used in the future to work around the missing isochronous transfer
support for WinUSB backends.

Here is the full changelog (from libusb).

2016-10-01: v1.0.21:
* Core: Refactor code related to transfer flags and timeout handling
* Darwin: Ignore root hub simulation devices
* Darwin: Improved support for OS X El Capitan
* Darwin: Work around devices with buggy endpoint descriptors
* Darwin: Do not use objc_registerThreadWithCollector after its deprecation
* Darwin: Use C11 atomics on 10.12+ as the OS atomics are now deprecated
* Linux: Support preallocating kernel memory for zerocopy USB
* Linux: Deal with receiving POLLERR before all transfers have completed
* Solaris: Add solaris backend
* Windows: Add Visual Studio 2015 support
* Windows: Add usbdk backend
* Prevent attempts to recursively handle events
* Fix race condition in handle_timeout()
* Allow transferred argument to be optional in bulk APIs
* Various other bug fixes and improvements

This reverts commit 7a305bef86e6e63eea2bee4024c5053a54d4e901.