uboot-imx

km83xx: use CONFIG_ENV_ADDR for the newenv env command

The hardcoded value are bad, since the address could change between
different boards.

Furthermore, the relevant #defines are set only if #undefined here, so
that they can be changed by some boards if required.

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