cmd_eeprom: I2C updates

- CONFIG_ENV_EEPROM_IS_ON_I2C
  define this, if you have I2C and SPI activated, and your
  EEPROM, which holds the environment, is on the I2C bus.

- CONFIG_I2C_ENV_EEPROM_BUS
  if you have an Environment on an EEPROM reached over
  I2C muxes, you can now define, how to reach this
  EEPROM.

Signed-off-by: Heiko Schocher <hs@denx.de>