added Boolean operators to HardwareSerial and CDC to test whether the port is ready to send data.

Mostly useful for Leonardo - simple way to test whether the port is actually opened by an application and ready to receive data.  For Serial objects attached to real UARTs always returns true.

hardware/arduino/cores/arduino/CDC.cpp

@@ -213,6 +213,12 @@ size_t Serial_::write(uint8_t c)
 	return 0;
 }
 
+Serial_::operator bool() {
+	if (_usbLineInfo.lineState > 0)
+		return true;
+	return false;
+}
+
 Serial_ Serial;
 
 #endif

hardware/arduino/cores/arduino/HardwareSerial.cpp

@@ -398,6 +398,10 @@ size_t HardwareSerial::write(uint8_t c)
   return 1;
 }
 
+HardwareSerial::operator bool() {
+	return true;
+}
+
 // Preinstantiate Objects //////////////////////////////////////////////////////
 
 #if defined(UBRRH) && defined(UBRRL)

hardware/arduino/cores/arduino/HardwareSerial.h

@@ -57,6 +57,7 @@ class HardwareSerial : public Stream
     virtual void flush(void);
     virtual size_t write(uint8_t);
     using Print::write; // pull in write(str) and write(buf, size) from Print
+    operator bool();
 };
 
 #if defined(UBRRH) || defined(UBRR0H)

hardware/arduino/cores/arduino/USBAPI.h

@@ -39,6 +39,7 @@ public:
 	virtual int read(void);
 	virtual void flush(void);
 	virtual size_t write(uint8_t);
+	operator bool();
 };
 extern Serial_ Serial;