More minor ESP8266 compatibility tweaks (#1131)

cores/rp2040/Arduino.h

@@ -107,6 +107,9 @@ extern bool __isFreeRTOS;
 
 #ifdef __cplusplus
 
+// emptyString is an ESP-ism, a constant string with ""
+extern const String emptyString;
+
 #ifdef USE_TINYUSB
 // Needed for declaring Serial
 #include "Adafruit_USBD_CDC.h"

cores/rp2040/main.cpp

@@ -195,3 +195,5 @@ void hexdump(const void* mem, uint32_t len, uint8_t cols) {
     }
     printf("\n");
 }
+
+const String emptyString = "";

libraries/WiFi/src/WiFiClass.h

@@ -381,6 +381,15 @@ public:
     void setFeedWatchdogFunc(FeedHostProcessorWatchdogFuncPointer func);
     void feedWatchdog();
 
+    // ESP8266 compatibility
+    void persistent(bool unused) {
+        (void) unused;
+    }
+
+    void hostname(const char *name) {
+        setHostname(name);
+    }
+
 private:
     int _timeout = 15000;
     String _ssid;