shutdown: add px4_shutdown_{lock,unlock} API methods

Prevents the system from shutting down.

src/platforms/px4_shutdown.h

@@ -81,5 +81,19 @@ __EXPORT int px4_unregister_shutdown_hook(shutdown_hook_t hook);
  */
 __EXPORT int px4_shutdown_request(bool reboot, bool to_bootloader);
 
+
+/**
+ * Grab the shutdown lock. It will prevent the system from shutting down until the lock is released.
+ * It is safe to call this recursively.
+ * @return 0 on success, <0 on error
+ */
+__EXPORT int px4_shutdown_lock(void);
+
+/**
+ * Release the shutdown lock.
+ * @return 0 on success, <0 on error
+ */
+__EXPORT int px4_shutdown_unlock(void);
+
 __END_DECLS