uboot-imx

MLK-22279-2 env: Add a config to enable nowhere as default location

When booting from USB there is no place to store the environment and
should use default env. However CONFIG_ENV_IS_NOWHERE has dependence
with other env storage config. If we enable multiple storages, NOWHERE
can't be enabled. And saveenv won't be built if NOWHERE is set.

This patch introduces a new CONFIG_ENV_DEFAULT_NOWHERE, that can enable
NOWHERE as a default env location when other storage are not avaliable.
And allow to build saveenv.

Signed-off-by: Ye Li <ye.li@nxp.com>
(cherry picked from commit 64c02d26555491eb7cb758f58511620c1590c378)