ARM: imx8m: support env in fat and ext4

Change boot device logic to also allow environment stored in fat and in
ext4 when booting from SD or from eMMC.

As the boot device check for SD and for eMMC was depending on
ENV_IS_IN_MMC being defined, change the ifdef blocks at env_get_location
to use IS_ENABLED instead for all modes, returning NOWHERE when no valid
mode is found.

Signed-off-by: Ricardo Salveti <ricardo@foundries.io>
Reviewed-by: Oleksandr Suvorov <oleksandr.suvorov@foundries.io>
Reviewed-by: Fabio Estevam <festevam@gmail.com>