Define correct size for RAM

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

Author: matthijskooijman

cores/arduino/avr/io.h

@@ -27,8 +27,8 @@
 
 #include "stm32l0xx.h"
 
-#define RAMSTART 0x20000000
-#define RAMSIZE  (20 * 1024)
+#define RAMSTART (SRAM_BASE)
+#define RAMSIZE  (SRAM_SIZE_MAX)
 #define RAMEND   (RAMSTART + RAMSIZE - 1)
 
 #if defined(DATA_EEPROM_BASE) && defined(DATA_EEPROM_END)