USB Host mass storage device example.

This example project uses the USB host and the MSD modules in the
drivers directory to implement support for Mass Storage Device's (MSD).

Connect a terminal to the DK UART port (115.200, N, 8, 1) and
then connect a memory stick the USB port of the MCU plug-in module.

Info about the MSD device will be output on the serial port. When a MSD
device is connected and detected as a valid, you can issue commands at the
serial port console to list all files on the device (ls), type the content
of a file (cat) and append test to a file (echo).

Note that USB disk drives may fail, as some of them draw too much current
when attached.

NOTE:
This example is too large to be built with IDEs with 32KB size limits.

Board:  Silicon Labs EFM32WG-DK3850 Development Kit
Device: EFM32WG990F256
