Use shared symbols for heap_ptr and heap_end

src/core/heap.c

 extern char _text[];
 extern char _end[];
 
-static physaddr_t heap_start, heap_end, heap_ptr;
+static physaddr_t heap_start;
 
 /*
  * Find the largest contiguous area of memory that I can use for the

src/include/heap.h

 	efree ( ptr );
 }
 
+/*
+ * Heap markers.  osloader.c and other code may wish to know the heap
+ * location, without necessarily wanting to drag in heap.o.  We
+ * therefore declare these as shared (i.e. common) symbols.
+ *
+ */
+physaddr_t heap_ptr __asm__ ( "_shared_heap_ptr" );
+physaddr_t heap_end __asm__ ( "_shared_heap_end" );
+
 #endif /* HEAP_H */