Fix HW Serial buffer size determination

Author: NicoHood

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

@@ -40,14 +40,14 @@
 // often work, but occasionally a race condition can occur that makes
 // Serial behave erratically. See https://github.com/arduino/Arduino/issues/2405
 #if !defined(SERIAL_TX_BUFFER_SIZE)
-#if (RAMEND < 1000)
+#if ((RAMEND - RAMSTART) < 1023)
 #define SERIAL_TX_BUFFER_SIZE 16
 #else
 #define SERIAL_TX_BUFFER_SIZE 64
 #endif
 #endif
 #if !defined(SERIAL_RX_BUFFER_SIZE)
-#if (RAMEND < 1000)
+#if ((RAMEND - RAMSTART) < 1023)
 #define SERIAL_RX_BUFFER_SIZE 16
 #else
 #define SERIAL_RX_BUFFER_SIZE 64