util/cbfstool: demote not-ELF error to debug

It's irritating when adding tianocore payloads - those are not
ELF, but that's deliberate.

Change-Id: I76d9367b28545348f526e5f0b8216f9ff2a3d636
Signed-off-by: Patrick Georgi <pgeorgi@google.com>
Reviewed-on: https://review.coreboot.org/20468
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Stefan Reinauer <stefan.reinauer@coreboot.org>

util/cbfstool/elfheaders.c

@@ -503,7 +503,7 @@ int parse_elf(const struct buffer *pinput, struct parsed_elf *pelf, int flags)
 	memset(pelf, 0, sizeof(*pelf));
 
 	if (!iself(buffer_get(pinput))) {
-		ERROR("The stage file is not in ELF format!\n");
+		DEBUG("The stage file is not in ELF format!\n");
 		return -1;
 	}