Adding USB MSD implementation to the HAL: Now, when you plug the Native USB...

Adding USB MSD implementation to the HAL: Now, when you plug the Native USB port of the Arduino DUE to your PC, it will be detected as a composite USB device: One of the devices is a USB CDC (Serial port over USB) with native USB handshake (no more overflows!!) (set SERIAL_PORT to -1 to use it as main serial port) and also, a MSD (USB Mass Storage Device) will be detected, and the SD card present on the socket will be reflected as a Removable disk on your PC)