Avoid "no config file" warning message

Set config to /dev/null when doing make rehash.
Reviewed-by: NRichard Levitte <levitte@openssl.org>

Makefile.org

@@ -438,8 +438,8 @@ rehash.time: certs apps
 	@if [ -z "$(CROSS_COMPILE)" ]; then \
 		(OPENSSL="`pwd`/util/opensslwrap.sh"; \
 		[ -x "apps/openssl.exe" ] && OPENSSL="apps/openssl.exe" || :; \
-		OPENSSL_DEBUG_MEMORY=on; \
-		export OPENSSL OPENSSL_DEBUG_MEMORY; \
+		OPENSSL_DEBUG_MEMORY=on; OPENSSL_CONFIG=/dev/null ; \
+		export OPENSSL OPENSSL_DEBUG_MEMORY OPENSSL_CONFIG; \
 		$(PERL) tools/c_rehash certs/demo) && \
 		touch rehash.time; \
 	else :; fi