Avoid issue: undefined reference to `vtable for HardwareSerial'

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

Author: fpistm

cores/arduino/HardwareSerial.h

@@ -26,8 +26,8 @@
 class HardwareSerial : public Stream
 {
   public:
-    virtual void begin(unsigned long);
-    virtual void end();
+    virtual void begin(unsigned long) = 0;
+    virtual void end() = 0;
     virtual int available(void) = 0;
     virtual int peek(void) = 0;
     virtual int read(void) = 0;