Print.flush() - Arduino API conformance (#6084)

cores/esp32/Print.h

@@ -108,6 +108,9 @@ public:
     size_t println(const Printable&);
     size_t println(struct tm * timeinfo, const char * format = NULL);
     size_t println(void);
+    
+    virtual void flush() { /* Empty implementation for backward compatibility */ }
+    
 };
 
 #endif

cores/esp32/Stream.h

@@ -48,7 +48,6 @@ public:
     virtual int available() = 0;
     virtual int read() = 0;
     virtual int peek() = 0;
-    virtual void flush() = 0;
 
     Stream():_startMillis(0)
     {