AP_HAL: ensure libc realloc is available for all platforms except ChibiOS

libraries/AP_HAL/AP_HAL_Boards.h

@@ -235,3 +235,7 @@
 #else
 #define AP_UAVCAN_SLCAN_ENABLED 0
 #endif
+
+#ifndef USE_LIBC_REALLOC
+#define USE_LIBC_REALLOC 1
+#endif

libraries/AP_HAL/Util.h

@@ -159,7 +159,11 @@ public:
     // heap functions, note that a heap once alloc'd cannot be dealloc'd
     virtual void *allocate_heap_memory(size_t size) = 0;
     virtual void *heap_realloc(void *heap, void *ptr, size_t new_size) = 0;
+#if USE_LIBC_REALLOC
     virtual void *std_realloc(void *ptr, size_t new_size) { return realloc(ptr, new_size); }
+#else
+    virtual void *std_realloc(void *ptr, size_t new_size) = 0;
+#endif // USE_LIBC_REALLOC
 #endif // ENABLE_HEAP