AP_HAL: added fs_init() to Util API

libraries/AP_HAL/Util.h

@@ -114,6 +114,12 @@ public:
        how much free memory do we have in bytes. If unknown return 4096
      */
     virtual uint32_t available_memory(void) { return 4096; }
+
+    /*
+      initialise (or re-initialise) filesystem storage
+     */
+    virtual bool fs_init(void) { return false; }
+
 protected:
     // we start soft_armed false, so that actuators don't send any
     // values until the vehicle code has fully started