follow arduino API: pure virtual Stream::flush() moved to empty virtual Print::flush() (#4029)

re fix #4018

Author: d-a-v

cores/esp8266/Print.h

@@ -89,6 +89,8 @@ class Print {
         size_t println(double, int = 2);
         size_t println(const Printable&);
         size_t println(void);
+
+        virtual void flush() { /* Empty implementation for backward compatibility */ }
 };
 
 #endif

cores/esp8266/Stream.h

@@ -47,7 +47,6 @@ class Stream: public Print {
         virtual int available() = 0;
         virtual int read() = 0;
         virtual int peek() = 0;
-        virtual void flush() = 0;
 
         Stream() {
             _timeout = 1000;