arm/km: use board KM_ENV_BUS for CONFIG_I2C_ENV_EEPROM_BUS

This is defined for all km_kirkwood boards and was not used up to now.
This value was the same for all boards but it could be changed for some
boards (and thus needs to be defined for every board).

Signed-off-by: Valentin Longchamp <valentin.longchamp@keymile.com>
Signed-off-by: Holger Brunck <holger.brunck@keymile.com>
cc: Prafulla Wadaskar <prafulla@marvell.com>
cc: Heiko Schocher <hs@denx.de>