USBXpress Echo example.

This example project demonstrates the EFM32 USBXpress firmware library.
It echoes all received data back to the host that sent it.

The host-side application is the usbxpress_echo.py Python script found in the
USBXpress SDK at Examples/Echo/Host. The USBXpress SDK is available for download
at www.silabs.com/products/mcu/Pages/USBXpress.aspx.

Board:  Silicon Labs EFM32LG_STK3600 Development Kit
Device: EFM32LG990F256
