diffconfig.sh runs ./scripts/config/conf, but it does not get built
with 'make {menu,x,n}config.  Call 'make ./scripts/config/conf' to
ensure it's been built before running it, aborting in case of failure.

Signed-off-by: Eneas U de Queiroz <cotequeiroz@gmail.com>
Signed-off-by: Petr Štetiar <ynezz@true.cz>[removed Fixes: due revert]