Update serial buffer size of the SAMD Core to 256

* Brings it inline with the change in Arduino's SAMD board files
arduino/ArduinoCore-samd@da9bfe8

Author: edspark

sparkfun/samd/cores/arduino/RingBuffer.h

@@ -27,7 +27,9 @@
 // using a ring buffer (I think), in which head is the index of the location
 // to which to write the next incoming character and tail is the index of the
 // location from which to read.
-#define SERIAL_BUFFER_SIZE 64
+#ifndef SERIAL_BUFFER_SIZE
+#define SERIAL_BUFFER_SIZE 256
+#endif
 
 template <int N>
 class RingBufferN