boot: image-pre-load: Check environment for location of signature info

Setting an alternative signature info node in "pre_load_sig_info_path"
allows verification of an image using the bootm pre-load mechanism with
a different key, e.g.: setenv pre_load_sig_info_path "/alt/sig" ; bootm
preload [addr]

Signed-off-by: Steven Lawrance <steven.lawrance@softathome.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/boot/image-pre-load.c b/boot/image-pre-load.c
index 01b6003..b504ab4 100644
--- a/boot/image-pre-load.c
+++ b/boot/image-pre-load.c
@@ -33,6 +33,7 @@
 	const u32 *sig_size;
 	int key_len;
 	int node, ret = 0;
+	char *sig_info_path = NULL;
 
 	if (!info) {
 		log_err("ERROR: info is NULL for image pre-load sig check\n");
@@ -42,7 +43,11 @@
 
 	memset(info, 0, sizeof(*info));
 
-	node = fdt_path_offset(gd_fdt_blob(), IMAGE_PRE_LOAD_PATH);
+	sig_info_path = env_get("pre_load_sig_info_path");
+	if (!sig_info_path)
+		sig_info_path = IMAGE_PRE_LOAD_PATH;
+
+	node = fdt_path_offset(gd_fdt_blob(), sig_info_path);
 	if (node < 0) {
 		log_info("INFO: no info for image pre-load sig check\n");
 		ret = 1;