tools: open FIT image read-only

Open for reading as O_RDONLY instead of O_RDWR:
the only usage of the fd is for the single read() below;
this prevented
	mkimage -f auto -A arm64 \
		-T kernel -C lz4 -d Image-6.6.15.lz4 \
		-b mt8173-elm-hana-6.6.15.dtb outf
when the inputs were unwritable.

Link: https://bugs.debian.org/1063097
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Reviewed-by: Dragan Simic <dsimic@manjaro.org>

tools/fit_image.c

@@ -137,7 +137,7 @@ static int fdt_property_file(struct image_tool_params *params,
 	int ret;
 	int fd;
 
-	fd = open(fname, O_RDWR | O_BINARY);
+	fd = open(fname, O_RDONLY | O_BINARY);
 	if (fd < 0) {
 		fprintf(stderr, "%s: Can't open %s: %s\n",
 			params->cmdname, fname, strerror(errno));