Added usb test plugin

This test plugin supports two modes: loopback, tx

In "loopback" mode the test plugins sends and receives data. In "tx"
mode data is only sent.

Various performance metrics are supported which can be read after a
successful run.

Also provided is an example test script (test-examples/usb/usb.lua).

To run the test script a USB loopback peripheral device is required.

The test script and plugin has been tested with a Cypress CY7C68013A
device loaded with a simple loopback firmware.