microbit: Automatic defining of SERIAL_DEBUG symbol

Added support for automatic creation and assignment of the SERIAL_DEBUG symbol
when MICORBIT_DBG is enabled in MicroBitConfig.h

Also reordered the MicroBit constructor to bring up the serial object before
all other objects, to enable debugging of all code.

inc/MicroBit.h

@@ -58,6 +58,9 @@ class MicroBit
 
     public:
 
+    // Serial Interface
+    MicroBitSerial              serial;
+
 	// Reset Button
 	InterruptIn     		    resetButton;
 
@@ -67,9 +70,6 @@ class MicroBit
     // I2C Interface
     MicroBitI2C                 i2c;
 
-    // Serial Interface
-    MicroBitSerial              serial;
-
     // Device level Message Bus abstraction
     MicroBitMessageBus          messageBus;
 

source/MicroBit.cpp

@@ -22,6 +22,10 @@
 #pragma GCC diagnostic pop
 #endif
 
+#if CONFIG_ENABLED(MICROBIT_DBG)
+// We create and initialize to NULL here, but MicroBitSerial will automatically update this as needed in its constructor.
+RawSerial* SERIAL_DEBUG = NULL;
+#endif
 
 /**
   * Constructor.
@@ -43,10 +47,10 @@
   * @endcode
   */
 MicroBit::MicroBit() :
+    serial(USBTX, USBRX),
 	resetButton(MICROBIT_PIN_BUTTON_RESET),
     storage(),
     i2c(I2C_SDA0, I2C_SCL0),
-    serial(USBTX, USBRX),
     messageBus(),
     display(),
     buttonA(MICROBIT_PIN_BUTTON_A, MICROBIT_ID_BUTTON_A),
@@ -201,3 +205,4 @@ void MicroBit::onListenerRegisteredEvent(MicroBitEvent evt)
     }
 }
 
+