Define correct size for EEPROM

This was previously hardcoded to a single value, but is now derived from
the STM32 chip-specific header files.

cores/arduino/avr/io.h

@@ -25,10 +25,19 @@
 #ifndef _IO_H_
 #define _IO_H_
 
+#include "stm32l0xx.h"
+
 #define RAMSTART 0x20000000
 #define RAMSIZE  (20 * 1024)
 #define RAMEND   (RAMSTART + RAMSIZE - 1)
 
-#define E2END    0xfff
+#if defined(DATA_EEPROM_BASE) && defined(DATA_EEPROM_END)
+  #define E2END (DATA_EEPROM_END - DATA_EEPROM_BASE)
+#elif defined(DATA_EEPROM_BASE) && defined(DATA_EEPROM_BANK2_BASE) && defined(DATA_EEPROM_BANK1_END) && defined(DATA_EEPROM_BANK2_END)
+  #define E2END (DATA_EEPROM_BANK1_END - DATA_EEPROM_BASE + 1 + DATA_EEPROM_BANK2_END - DATA_EEPROM_BANK2_BASE)
+#else
+  #error "Cannot determine EEPROM size"
+#endif
+
 
 #endif