EEPROM: Improve interface and implementation

[mikaelpatel]

Consider 1) using eeprom_update() instead of eeprom_write(), 2) add function to return EEPROM size, 3) using eeprom_update_block() and eeprom_read_block() instead of implementing them.

Use available constants (e.g. from iom328p.h):

#define RAMSTART     (0x100)
#define RAMEND       0x8FF     /* Last On-Chip SRAM Location */
#define XRAMSIZE     0
#define XRAMEND      RAMEND
#define E2END        0x3FF
#define E2PAGESIZE   4
#define FLASHEND     0x7FFF

E2END+1 is the size of the EEPROM (1 Kbyte). RAMEND+1-RAMSTART is the size of the SRAM (2 Kbyte).