uboot-imx

MLK-22287 imx8mn: enable CONFIG_ENV_DEFAULT_NOWHERE at default

This CONFIG_ENV_DEFAULT_NOWHERE config must be enabled for USB download
when we implemented the override function to select env storage medium.
To avoid adding it to every imx8mn defconfig, enable the config as default
on imx8mn.

Signed-off-by: Ye Li <ye.li@nxp.com>
(cherry picked from commit 3f94cf98113af6c31e146afcf394f34ff4d2bc00)