KM: add CONFIG_KM_COMMON_ETH_INIT for km common eth init

This must be defined by a board support file that want to use the
keymile common.c board_eth_init function that requires ethernet_present
to be defined.

Currently all the km architectures use it but the kmp204x architecture
later supported in this series does use another board_eth_init function
and thus does not define it.

Signed-off-by: Valentin Longchamp <valentin.longchamp@keymile.com>