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