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.

src/main.h

@@ -1,8 +1,9 @@
 #ifndef MAIN_H
 #include "config.h"
 #include <Arduino.h>
+#include <ComPort.h>
 #ifdef DEBUG_ON
-#define D(x) Serial.println(x)
+#define D(x) comPort->println(x)
 #else
 #define D(x)
 #endif