HardwareSerial to be exported

Author: earlephilhower

api/HardwareSerial.h

@@ -103,4 +103,6 @@ class HardwareSerial : public Stream
 // XXX: Are we keeping the serialEvent API?
 extern void serialEventRun(void) __attribute__((weak));
 
-}
+}
+
+using arduino::HardwareSerial;