test: Tweak FDT-overlay tests

Use fdt_overlay consistently in the identifiers and file/dir names.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/test/Kconfig b/test/Kconfig
index 1c8e3d1..01d6464 100644
--- a/test/Kconfig
+++ b/test/Kconfig
@@ -120,7 +120,7 @@
 source "test/image/Kconfig"
 source "test/lib/Kconfig"
 source "test/optee/Kconfig"
-source "test/overlay/Kconfig"
+source "test/fdt_overlay/Kconfig"
 
 config POST
 	bool "Power On Self Test support"