esp8266: Provide a dedicated variable to disable ets_loop_iter.

So ets_loop_iter is now only disabled when using machine.disable_irq.
diff --git a/esp8266/ets_alt_task.h b/esp8266/ets_alt_task.h
index c423995..4b5ba26 100644
--- a/esp8266/ets_alt_task.h
+++ b/esp8266/ets_alt_task.h
@@ -1 +1,2 @@
+extern int ets_loop_iter_disable;
 bool ets_loop_iter(void);