configs: sandbox64: Enable legacy image format support

Align the sandbox64 defconfig with sandbox defconfig. Enable missing
legacy image format support. This fixes ut_bootstd_bootflow_cmd_menu
test.

Suggested-by: Jonas Karlman <jonas@kwiboo.se>
Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>
diff --git a/configs/sandbox64_defconfig b/configs/sandbox64_defconfig
index 55a01b7..b973805 100644
--- a/configs/sandbox64_defconfig
+++ b/configs/sandbox64_defconfig
@@ -13,6 +13,7 @@
 CONFIG_FIT=y
 CONFIG_FIT_SIGNATURE=y
 CONFIG_FIT_VERBOSE=y
+CONFIG_LEGACY_IMAGE_FORMAT=y
 CONFIG_DISTRO_DEFAULTS=y
 CONFIG_BOOTSTAGE=y
 CONFIG_BOOTSTAGE_REPORT=y