Add flush() to Print for backwards compatibility

Author: Bryan Hatasaka

cores/nRF5/Print.h

@@ -78,6 +78,8 @@ class Print
     size_t println(double, int = 2);
     size_t println(const Printable&);
     size_t println(void);
+    
+    virtual void flush() { /* Empty for backward compatibility */ }
 };
 
 #endif