Merge in com port switching from sangaboard v4

I've refactored the use of the serial port, to store the
current serial port in a global variable in a module.
This is set to the most recent port to receive data,
and so we should always be replying to the port
that was used to initiate a command.

If SECONDARY_SERIAL_PORT is not defined, this
should result in no change - if it is, we should be
able to communicate on either port.

lib/ComPort/ComPort.h

@@ -0,0 +1,9 @@
+#ifndef COMPORT_H
+#define COMPORT_H
+
+#include <Arduino.h>
+
+// comPort is a Serial-like object that we can switch.
+extern HardwareSerial* comPort;
+
+#endif