AP_HAL: added optional healthy check on storage object

used to check parameter storage is functional

libraries/AP_HAL/Storage.h

@@ -9,4 +9,5 @@ public:
     virtual void read_block(void *dst, uint16_t src, size_t n) = 0;
     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; }
 };