boot/image-board.c: Silence warning in select_ramdisk

When building with clang we get a warning that rdaddr could be
uninitialized in one case. While this cannot functionally happen, we can
easily silence the warning.

Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/boot/image-board.c b/boot/image-board.c
index c602832..d500da1 100644
--- a/boot/image-board.c
+++ b/boot/image-board.c
@@ -328,7 +328,7 @@
 	bool done_select = !select;
 	bool done = false;
 	int rd_noffset;
-	ulong rd_addr;
+	ulong rd_addr = 0;
 	char *buf;
 
 	if (CONFIG_IS_ENABLED(FIT)) {