Implement esp_yield() as a replacement for delay(0)

esp_yield() now also calls esp_schedule(), original esp_yield() function renamed to esp_suspend().

Don't use delay(0) in the Core internals, libraries and examples. Use yield() when the code is
supposed to be called from CONT, use esp_yield() when the code can be called from either CONT or SYS.
Clean-up esp_yield() and esp_schedule() declarations across the code and use coredecls.h instead.

Implement helper functions for libraries that were previously using esp_yield(), esp_schedule() and
esp_delay() directly to wait for certain SYS context tasks to complete. Correctly use esp_delay()
for timeouts, make sure scheduled functions have a chance to run (e.g. LwIP_Ethernet uses recurrent)

Related issues:
- #6107 - discussion about the esp_yield() and esp_delay() usage in ClientContext
- #6212 - discussion about replacing delay() with a blocking loop
- #6680 - pull request introducing LwIP-based Ethernet
- #7146 - discussion that originated UART code changes
- #7969 - proposal to remove delay(0) from the example code
- #8291 - discussion related to the run_scheduled_recurrent_functions() usage in LwIP Ethernet
- #8317 - yieldUntil() implementation, similar to the esp_delay() overload with a timeout and a 0 interval

libraries/ESP8266HTTPClient/src/ESP8266HTTPClient.cpp

@@ -22,6 +22,7 @@
  *
  */
 #include <Arduino.h>
+#include <coredecls.h>
 
 #include "ESP8266HTTPClient.h"
 #include <ESP8266WiFi.h>
@@ -556,6 +557,7 @@ int HTTPClient::sendRequest(const char * type, Stream * stream, size_t size)
     if (transferred != size)
     {
         DEBUG_HTTPCLIENT("[HTTP-Client][sendRequest] short write, asked for %d but got %d failed.\n", size, transferred);
+        esp_yield();
         return returnError(HTTPC_ERROR_SEND_PAYLOAD_FAILED);
     }
 
@@ -698,7 +700,7 @@ int HTTPClient::writeToPrint(Print * print)
                 return returnError(HTTPC_ERROR_READ_TIMEOUT);
             }
 
-            delay(0);
+            esp_yield();
         }
     } else {
         return returnError(HTTPC_ERROR_ENCODING);
@@ -1060,7 +1062,7 @@ int HTTPClient::handleHeaderResponse()
             if((millis() - lastDataTime) > _tcpTimeout) {
                 return HTTPC_ERROR_READ_TIMEOUT;
             }
-            delay(0);
+            esp_yield();
         }
     }