Align with Version 1.6.5

Update to match Version 1.6.5.

Author: MHeironimus

hardware/arduino/avr/cores/arduino/USBAPI.h

@@ -59,11 +59,16 @@ extern USBDevice_ USBDevice;
 
 struct ring_buffer;
 
+#ifndef SERIAL_BUFFER_SIZE
 #if (RAMEND < 1000)
 #define SERIAL_BUFFER_SIZE 16
 #else
 #define SERIAL_BUFFER_SIZE 64
 #endif
+#endif
+#if (SERIAL_BUFFER_SIZE>256)
+#error Please lower the CDC Buffer size
+#endif
 
 class Serial_ : public Stream
 {