test: Load mac address with i2c eeprom

This uses an i2c eeprom to load a mac address using the nvmem interface.
Enable I2C_EEPROM for sandbox SPL since it is the only sandbox config
which doesn't enable it eeprom.

Signed-off-by: Sean Anderson <sean.anderson@seco.com>
Reviewed-by: Simon Glass <sjg@chromium.org>