Add default EEPROM read/write routines

Reviewed By: dustin

Add common EEPROM (or board info) read/write routines to the core
sys_eeprom command.

Many platforms use one of two common methods for storing board
information:

- An EEPROM accessible via I2C
- A sector in the NOR flash

This patch adds both of these methods.  Use the following #define's to
enable a particular method:

CONFIG_SYS_EEPROM_USE_COMMON_FLASH_IO:

  This method also requires the following constants:

     CONFIG_SYS_FLASH_HWINFO_ADDR
     CONFIG_SYS_FLASH_HWINFO_SECT_SIZE

CONFIG_SYS_EEPROM_USE_COMMON_I2C_IO:

  This method also requires the following constants:

    CONFIG_SYS_I2C_EEPROM_ADDR
    CONFIG_SYS_I2C_EEPROM_ADDR_LEN
    CONFIG_SYS_EEPROM_PAGE_WRITE_DELAY_MS