Add freeRam() function for stm32

speeduino/globals.h

@@ -13,6 +13,7 @@
   #define CORE_STM32
   #define LED_BUILTIN 33
 
+  extern "C" char* sbrk(int incr); //Used to freeRam
   inline unsigned char  digitalPinToInterrupt(unsigned char Interrupt_pin) { return Interrupt_pin; } //This isn't included in the stm32duino libs (yet)
   #define portOutputRegister(port) (volatile byte *)( &(port->regs->ODR) ) //These are defined in STM32F1/variants/generic_stm32f103c/variant.h but return a non byte* value
   #define portInputRegister(port) (volatile byte *)( &(port->regs->IDR) ) //These are defined in STM32F1/variants/generic_stm32f103c/variant.h but return a non byte* value

speeduino/utils.ino

@@ -34,8 +34,9 @@ uint16_t freeRam ()
   // The difference is the free, available ram.
   return (uint16_t)stackTop - heapTop;
 #elif defined(CORE_STM32)
-  //Figure this out some_time
-  return 0;
+  char top = 't';
+  return &top - reinterpret_cast<char*>(sbrk(0));
+
 #endif
 }