Add loopback function. This function can be used to test
USB device drivers and device stack connected to linux host
and has the similar interface as "Gadget Zero" [1] of the Linux
kernel.
Use modprobe usbtest to load the module, see also [2] for the
description of the tests and for Vendor and Product ID of the
"Gadget Zero". The userspace tool testusb [3] is needed to start
the tests.
[1] linux/drivers/usb/gadget/function/f_loopback.c
[2] linux/drivers/usb/misc/usbtest.c
[3] linux/tools/usb/testusb.c
Signed-off-by: Johann Fischer <j.fischer@phytec.de>
