Keep Serial from being linked when disabled

Even with MY_DISABLED_SERIAL defined, the linker is still adding the
Serial object to the binary.

That's because a) the MySensors *.cpp files are actually included
into the sketch and compiled as part of it (and not compiled
separately and then linked) and b) due the the default(?) behavior of
the linker (https://github.com/arduino/Arduino/issues/4579).

Author: rclasen

hal/architecture/MyHwAVR.cpp

@@ -301,6 +301,9 @@ uint16_t hwFreeMem()
 	return (int) &v - (__brkval == 0 ? (int) &__heap_start : (int) __brkval);
 }
 
+#ifdef MY_DISABLED_SERIAL
+#define hwDebugPrint(fmt,...)
+#else
 void hwDebugPrint(const char *fmt, ... )
 {
 	char fmtBuffer[MY_SERIAL_OUTPUT_SIZE];
@@ -328,3 +331,4 @@ void hwDebugPrint(const char *fmt, ... )
 	MY_SERIALDEVICE.print(fmtBuffer);
 	MY_SERIALDEVICE.flush();
 }
+#endif