Add private availableForStore method, and use in Serial_::accept

cores/arduino/USB/CDC.cpp

@@ -351,6 +351,17 @@ bool Serial_::rts() {
 	return _usbLineInfo.lineState & 0x2;
 }
 
+int Serial_::availableForStore(void) {
+	ring_buffer *buffer = &cdc_rx_buffer;
+
+	if (buffer->full)
+		return 0;
+	else if (buffer->head >= buffer->tail)
+		return CDC_SERIAL_BUFFER_SIZE - 1 - buffer->head + buffer->tail;
+	else
+		return buffer->tail - buffer->head  - 1;
+}
+
 Serial_ SerialUSB(USBDevice);
 
 #endif

cores/arduino/USB/USBAPI.h

@@ -168,6 +168,8 @@ public:
 	};
 
 private:
+	int availableForStore(void);
+
 	USBDeviceClass &usb;
 	RingBuffer *_cdc_rx_buffer;
 };