fix(cert-create): change WARN to VERBOSE

SAVE_KEYS is set to '0' by default, causing cert_create to
show the 'Key filename not specified' message on each run
even though this is perfectly normal. Show the message only
in the VERBOSE log level.

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Change-Id: I472cdec2670055ab0edd99d172f79d01ad575972
diff --git a/tools/cert_create/src/key.c b/tools/cert_create/src/key.c
index 487777b..27ec979 100644
--- a/tools/cert_create/src/key.c
+++ b/tools/cert_create/src/key.c
@@ -212,7 +212,7 @@
 			*err_code = KEY_ERR_OPEN;
 		}
 	} else {
-		WARN("Key filename not specified\n");
+		VERBOSE("Key filename not specified\n");
 		*err_code = KEY_ERR_FILENAME;
 	}