arm/km: rename CONFIG option CONFIG_KM_DEF_ENV_UPDATE

This config option sounds like the it is responsible for the
update of  the environment, but it is the u-boot update handling.
Therefore we adapt it to a more apropriate naming.

Signed-off-by: Holger Brunck <holger.brunck@keymile.com>
Signed-off-by: Prafulla Wadaskar <prafulla@marvell.com>
Signed-off-by: Valentin Longchamp <valentin.longchamp@keymile.com>
cc: Gerlando Falauto <gerlando.falauto@keymile.com>
cc: Prafulla Wadaskar <prafulla@marvell.com>