scripts/diffconfig.sh: ensure config/conf is built

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]

scripts/diffconfig.sh

 #!/bin/sh
+make ./scripts/config/conf >/dev/null || { make ./scripts/config/conf; exit 1; }
 grep \^CONFIG_TARGET_ .config | head -n3 > tmp/.diffconfig.head
 grep \^CONFIG_TARGET_DEVICE_ .config >> tmp/.diffconfig.head
 grep '^CONFIG_ALL=y' .config >> tmp/.diffconfig.head