microbit: fix for #73

The destructor for MicroBitButton didn't take into account the fact that
an instance can be destructed after registering a callback. This would then
result in a HardFault if an instance was destructed.

A clear repro case was registering a Touch input on a pin (abstracted as a button)
and swapping to a DigitalIn.

inc/MicroBitButton.h

@@ -96,6 +96,10 @@ class MicroBitButton : public MicroBitComponent
       */
     virtual void systemTick();
 
+    /**
+      * Destructor for MicroBitButton, so that we deregister ourselves as a systemComponent
+      */
+    ~MicroBitButton();
 };
 
 #endif

source/MicroBitButton.cpp

@@ -100,3 +100,11 @@ int MicroBitButton::isPressed()
 {
     return status & MICROBIT_BUTTON_STATE ? 1 : 0;
 }
+
+/**
+  * Destructor for MicroBitButton, so that we deregister ourselves as a systemComponent
+  */
+MicroBitButton::~MicroBitButton()
+{
+    uBit.removeSystemComponent(this);
+}