board/km: remove CONFIG_KM_BOARD_NAME

We can use the existing CONFIG_SYS_CONFIG_NAME define for that and
remove the option. Also fix the boot string for all km83xx boards.

Signed-off-by: Holger Brunck <holger.brunck@ch.abb.com>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>
CC: Priyanka Jain <priyanka.jain@nxp.com>
CC: Valentin Longchamp <valentin.longchamp@ch.abb.com>