Moved Stream.flush() to Print.flush()

Fix stm32duino#1170

Signed-off-by: Frederic Pillon <frederic.pillon@st.com>

Author: fpistm

cores/arduino/Print.h

@@ -99,6 +99,8 @@ class Print {
 
     int printf(const char *format, ...);
     int printf(const __FlashStringHelper *format, ...);
+
+    virtual void flush() { /* Empty implementation for backward compatibility */ }
 };
 
 #endif

cores/arduino/Stream.h

@@ -58,7 +58,6 @@ class Stream : public Print {
     virtual int available() = 0;
     virtual int read() = 0;
     virtual int peek() = 0;
-    virtual void flush() = 0;
 
     Stream()
     {