image: Rename load_addr, save_addr, save_size

These global variables are quite short and generic. In fact the same name
is more often used locally for struct members and function arguments.

Add a image_ prefix to make them easier to distinguish.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/cmd/nand.c b/cmd/nand.c
index 5bda69e..92089a7 100644
--- a/cmd/nand.c
+++ b/cmd/nand.c
@@ -925,7 +925,7 @@
 
 	/* Loading ok, update default load address */
 
-	load_addr = addr;
+	image_load_addr = addr;
 
 	return bootm_maybe_autostart(cmdtp, cmd);
 }