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/ESP8266WiFiMulti.cpp

@@ -25,6 +25,7 @@
 
 #include "PolledTimeout.h"
 #include "ESP8266WiFiMulti.h"
+#include <coredecls.h>
 #include <limits.h>
 #include <string.h>
 
@@ -82,37 +83,29 @@ static void printWiFiStatus(wl_status_t status)
  */
 static wl_status_t waitWiFiConnect(uint32_t connectTimeoutMs)
 {
-    wl_status_t status;
+    wl_status_t status = WL_CONNECT_FAILED;
+    // The final argument, intvl_ms, to esp_delay influences how frequently
+    // the scheduled recurrent functions (Schedule.h) are probed.
+    esp_delay(connectTimeoutMs,
+        [&status]() {
+            status = WiFi.status();
+            return status != WL_CONNECTED && status != WL_CONNECT_FAILED;
+        }, 0);
 
-    // Set WiFi connect timeout
-    using esp8266::polledTimeout::oneShotMs;
-    oneShotMs connectTimeout(connectTimeoutMs);
+    // Check status
+    if (status == WL_CONNECTED) {
+        // Connected, print WiFi status
+        printWiFiStatus(status);
 
-    // Wait for WiFi status change or timeout
-    do {
-        // Refresh watchdog
-        delay(0);
-
-        // Get WiFi status
-        status = WiFi.status();
-
-        // Check status
-        if (status == WL_CONNECTED) {
-            // Connected, print WiFi status
-            printWiFiStatus(status);
-
-            // Return WiFi status
-            return status;
-        } else if (status == WL_CONNECT_FAILED) {
-            DEBUG_WIFI_MULTI("[WIFIM] Connect failed\n");
-
-            // Return WiFi connect failed
-            return WL_CONNECT_FAILED;
-        }
-    } while (!connectTimeout);
-
-    DEBUG_WIFI_MULTI("[WIFIM] Connect timeout\n");
+        // Return WiFi status
+        return status;
+    } else if (status == WL_CONNECT_FAILED) {
+        DEBUG_WIFI_MULTI("[WIFIM] Connect failed\n");
+    } else {
+        DEBUG_WIFI_MULTI("[WIFIM] Connect timeout\n");
+    }
 
+    // Return WiFi connect failed
     return WL_CONNECT_FAILED;
 }
 
@@ -242,24 +235,19 @@ int8_t ESP8266WiFiMulti::startScan()
     // Start wifi scan in async mode
     WiFi.scanNetworks(true);
 
-    // Set WiFi scan timeout
-    using esp8266::polledTimeout::oneShotMs;
-    oneShotMs scanTimeout(WIFI_SCAN_TIMEOUT_MS);
-
     // Wait for WiFi scan change or timeout
-    do {
-        // Refresh watchdog
-        delay(0);
-
-        // Check scan timeout which may occur when scan does not report completion
-        if (scanTimeout) {
-            DEBUG_WIFI_MULTI("[WIFIM] Scan timeout\n");
-            return WIFI_SCAN_FAILED;
-        }
-
-        // Get scan result
-        scanResult = WiFi.scanComplete();
-    } while (scanResult < 0);
+    // The final argument, intvl_ms, to esp_delay influences how frequently
+    // the scheduled recurrent functions (Schedule.h) are probed.
+    esp_delay(WIFI_SCAN_TIMEOUT_MS,
+        [&scanResult]() {
+            scanResult = WiFi.scanComplete();
+            return scanResult < 0;
+        }, 0);
+    // Check for scan timeout which may occur when scan does not report completion
+    if (scanResult < 0) {
+        DEBUG_WIFI_MULTI("[WIFIM] Scan timeout\n");
+        return WIFI_SCAN_FAILED;
+    }
 
     // Print WiFi scan result
     printWiFiScan();
@@ -535,7 +523,7 @@ void ESP8266WiFiMulti::printWiFiScan()
                          rssi,
                          (encryptionType == ENC_TYPE_NONE) ? ' ' : '*',
                          ssid.c_str());
-        delay(0);
+        esp_yield();
     }
 #endif
 }