AP_HAL: add get_storage_ptr method

libraries/AP_HAL/Storage.h

@@ -11,4 +11,5 @@ public:
     virtual void write_block(uint16_t dst, const void* src, size_t n) = 0;
     virtual void _timer_tick(void) {};
     virtual bool healthy(void) { return true; }
+    virtual bool get_storage_ptr(void *&ptr, size_t &size) { return false; }
 };