Correct undefined Serial Buffer Size

https://github.com/arduino/Arduino/pull/2597

.gitignore

@@ -1,2 +1 @@
-dev/*
-*.pdn
+dev/*

avr/cores/hid/HardwareSerial.h

@@ -34,12 +34,17 @@
 // location from which to read.
 // NOTE: a "power of 2" buffer size is reccomended to dramatically
 //       optimize all the modulo operations for ring buffers.
-#if !(defined(SERIAL_TX_BUFFER_SIZE) && defined(SERIAL_RX_BUFFER_SIZE))
+#if !(defined(SERIAL_TX_BUFFER_SIZE)
 #if (RAMEND < 1000)
 #define SERIAL_TX_BUFFER_SIZE 16
-#define SERIAL_RX_BUFFER_SIZE 16
 #else
 #define SERIAL_TX_BUFFER_SIZE 64
+#endif
+#endif
+#if !defined(SERIAL_RX_BUFFER_SIZE))
+#if (RAMEND < 1000)
+#define SERIAL_RX_BUFFER_SIZE 16
+#else
 #define SERIAL_RX_BUFFER_SIZE 64
 #endif
 #endif

avr/cores/hid/USB-Core/CDC.h

@@ -56,11 +56,16 @@ THE SOFTWARE.
 
 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
 {