util/sconfig: fix check for count of command line arguments

Valid invocations are when -s|b|k outputfile is missing (argc == 3)
and when it is followed by the file name (argc == 5); it's an error
when "outputfile" is missing (argc == 4) or when there are more
arguments than expected (argc > 5).
Fixes "Uninitialized argument value" error found by scan-build from
clang version 3.2-11.

Change-Id: I8c489863323eb60cbaa5e82a80f5d78a6ca893c2
Signed-off-by: Daniele Forsi <dforsi@gmail.com>
Reviewed-on: http://review.coreboot.org/6378
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>

util/sconfig/main.c

@@ -557,7 +557,7 @@ static void usage(void)
 
 
 int main(int argc, char** argv) {
-	if (argc < 3)
+	if (argc != 3 && argc != 5)
 		usage();
 
 	char *mainboard=argv[1];