smart-gateway-mt7688: Rework build time check for overwriting factory data

The board file has a build time check to ensure that we do not have the
redundant environment overwriting the factory data.  However, using the
symbol CONFIG_ENV_SIZE_REDUND isn't strictly needed as that is always
the same as CONFIG_ENV_SIZE.  Use CONFIG_ENV_SIZE instead so that we can
later drop CONFIG_ENV_SIZE_REDUND.

Cc: Stefan Roese <sr@denx.de>
Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Stefan Roese <sr@denx.de>