Re-enabled serial interrupt handling.
Changed sdcard directory.
Handle usbserial disconnects.
Set gcode_LastN only if we got a command with a line number.
UsbCommand is now a class - with constructor and reset method. Reset
UsbCommand counters on usbserial errors. Added usbserial RX timeout.
Added synchronize mode, this mode is entered after an error to prevent
blocking reads in the firmware (for example if a terminating newline
gets lost). Empty commands (newlines) are sent until a acknowledge is
received.