env: Add !ENV_IS_IN_EXT4 dependency to ENV_IS_NOWHERE
If ENV_IS_IN_EXT4 is set you shouldn't be able to select ENV_IS_NOWHERE.
Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Tested-by: Petr Vorel <petr.vorel@gmail.com>