Add Serial_::availableForWrite().

This makes the CDC "Serial" object on the Leonardo and similar boards
support this recently introduced method as well. The CDC code in the sam
core is not changed.

Author: Peter Van Hoyweghen

Diff for: hardware/arduino/avr/cores/arduino/CDC.cpp

@@ -163,6 +163,11 @@ int Serial_::read(void)
 	return USB_Recv(CDC_RX);
 }
 
+int Serial_::availableForWrite(void)
+{
+	return USB_SendSpace(CDC_TX);
+}
+
 void Serial_::flush(void)
 {
 	USB_Flush(CDC_TX);

Diff for: hardware/arduino/avr/cores/arduino/USBAPI.h

@@ -92,6 +92,7 @@ class Serial_ : public Stream
 	virtual int available(void);
 	virtual int peek(void);
 	virtual int read(void);
+	int availableForWrite(void);
 	virtual void flush(void);
 	virtual size_t write(uint8_t);
 	virtual size_t write(const uint8_t*, size_t);
@@ -188,6 +189,7 @@ int USB_SendControl(uint8_t flags, const void* d, int len);
 int USB_RecvControl(void* d, int len);
 
 uint8_t	USB_Available(uint8_t ep);
+uint8_t USB_SendSpace(uint8_t ep);
 int USB_Send(uint8_t ep, const void* data, int len);	// blocking
 int USB_Recv(uint8_t ep, void* data, int len);		// non-blocking
 int USB_Recv(uint8_t ep);							// non-blocking