test/overlay: Make this depend on SANDBOX

As the overlays are sandbox specific, mark the whole test as depending
on sandbox.

Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/test/overlay/Kconfig b/test/overlay/Kconfig
index a4f1544..8818489 100644
--- a/test/overlay/Kconfig
+++ b/test/overlay/Kconfig
@@ -1,6 +1,6 @@
 config UT_OVERLAY
 	bool "Enable Device Tree Overlays Unit Tests"
-	depends on UNIT_TEST && OF_CONTROL
+	depends on UNIT_TEST && OF_CONTROL && SANDBOX
 	default y
 	select OF_LIBFDT_OVERLAY
 	help