AP_Logger: process pending rotate on arming

If the user arms the vehicle during the logging-persist-timeout we
should rotate the log immediately.

libraries/AP_Logger/AP_Logger_File.cpp

@@ -767,6 +767,10 @@ void AP_Logger_File::stop_logging(void)
 
 void AP_Logger_File::PrepForArming()
 {
+    if (_rotate_pending) {
+        _rotate_pending = false;
+        stop_logging();
+    }
     if (logging_started()) {
         return;
     }