Falcon mode uses falcon_image_file from the env during mmc fs boot, but
external env can be compromised. Therefore disable access to external
env by setting SPL_ENV_IS_NOWHERE when SPL_OS_BOOT_SECURE is set.
Signed-off-by: Anshul Dalal <anshuld@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>