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/ESP8266WiFi/src/ESP8266WiFiSTA.cpp

@@ -28,6 +28,8 @@
 #include "PolledTimeout.h"
 #include "LwipIntf.h"
 
+#include <coredecls.h>
+
 #include "c_types.h"
 #include "ets_sys.h"
 #include "os_type.h"
@@ -44,9 +46,6 @@ extern "C" {
 
 #include "debug.h"
 
-extern "C" void esp_schedule();
-extern "C" void esp_yield();
-
 // -----------------------------------------------------------------------------------------------------------------------
 // ---------------------------------------------------- Private functions ------------------------------------------------
 // -----------------------------------------------------------------------------------------------------------------------