Fix: WiFiClient::flush() yields but can be called from events (#5254)

Fix bug introduced by #5167 which replaced delay() by yield().
That should have been esp_yield() which is the one delay()
calls and is safe from either SYS or CONT contexts.

Fixes #5237.

libraries/ESP8266WiFi/src/include/ClientContext.h

@@ -333,7 +333,7 @@ class ClientContext
                 last_sent = millis();
             }
 
-            yield();
+            esp_yield(); // from sys or os context
 
             if ((state() != ESTABLISHED) || (sndbuf == TCP_SND_BUF)) {
                 break;