cpu/esp8266: fix ets_post when flash is written

During flash write access, the IROM cache cannot be used and is disabled therefore. During that time, ets_post crashes if a functions is called which is not in IRAM. Therefore thread_flags_set must not be called if IROM cache is disabled.

cpu/esp8266/sdk/ets_task.c

@@ -181,7 +181,7 @@ uint32_t IRAM ets_post (uint32_t prio, ETSSignal sig, ETSParam par)
         system_soft_wdt_feed();
     }
 
-    if (ets_thread) {
+    if (ets_thread && irom_cache_enabled()) {
         thread_flags_set((thread_t*)ets_thread, THREAD_FLAG_ETS_THREAD);
     }