Revert "board: st: stm32mp1: Clean env_get_location()"

Revert "board: st: stm32mp1: Clean env_get_location()"

This reverts commit d37641c61ba212241c38e3fd22f0335fc0bebc96.

Restore support for environment storage in EXT4 filesystem on eMMC boot.
The previous cleanup incorrectly removed this fallback option which is
needed for boards that store their environment in an EXT4 partition.

This configuration is OS-specific rather than board-dependent and should
remain as it is configurable via menuconfig. Even if it is not described
in ST defconfigs people may have enabled it in their defconfig.

Signed-off-by: Kory Maincent <kory.maincent@bootlin.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>