Print hangs if PC stops accepting USB data

[dc42]

If a print is started with the USB cable connected, under certain conditions the PC will stop accepting data over the USB connection, e.g. if it goes to sleep, or if Pronterface is closed, or sometimes when another USB device is connected, or if the USB cable is disconnected. When this happens, if the firmware tries to send data to the USB, it will block. This causes the main spin loop to stop executing. Not only does this pause the print, it also causes the heaters to become uncontrolled.

Fixed in dc42 fork.