Weak binding lessens version dependency between ESP8266 and MT library CoopTask. (#6790)

Author: dok-net

cores/esp8266/core_esp8266_main.cpp

@@ -100,12 +100,14 @@ static void esp_yield_within_cont() {
         run_scheduled_recurrent_functions();
 }
 
-extern "C" void esp_yield() {
+extern "C" void __esp_yield() {
     if (can_yield()) {
         esp_yield_within_cont();
     }
 }
 
+extern "C" void esp_yield() __attribute__ ((weak, alias("__esp_yield")));
+
 extern "C" void esp_schedule() {
     // always on CONT stack here
     ets_post(LOOP_TASK_PRIORITY, 0, 0);