AP_HAL: add util method boot_to_dfu()

libraries/AP_HAL/Util.h

@@ -81,6 +81,7 @@ public:
         int8_t scheduler_task;
         bool armed; // true if vehicle was armed
         enum safety_state safety_state;
+        bool boot_to_dfu; // true if we should reboot to DFU on boot
     };
     struct PersistentData persistent_data;
     // last_persistent_data is only filled in if we've suffered a watchdog reset
@@ -201,6 +202,9 @@ public:
     virtual void* last_crash_dump_ptr() const { return nullptr; }
 #endif
 
+#if HAL_ENABLE_DFU_BOOT
+    virtual void boot_to_dfu(void) {}
+#endif
 protected:
     // we start soft_armed false, so that actuators don't send any
     // values until the vehicle code has fully started