fiptool: fix existence check of FIP input file for update command

This line should check the existence of the input file, but it is
actually checking the output file.  When -o option is given to the
"update" command, the outfile is unlikely to exist, then parse_fip()
is skipped and an empty FIP file is output.  This is wrong behavior.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

tools/fiptool/fiptool.c

@@ -882,7 +882,7 @@ static int update_cmd(int argc, char *argv[])
 	if (outfile[0] == '\0')
 		snprintf(outfile, sizeof(outfile), "%s", argv[0]);
 
-	if (access(outfile, F_OK) == 0)
+	if (access(argv[0], F_OK) == 0)
 		parse_fip(argv[0], &toc_header);
 
 	if (pflag)