tests: Introduce DT overlay tests

This adds a bunch of unit tests for the "fdt apply" command.

They've all been run successfully in the sandbox. However, as you still
require an out-of-tree dtc with overlay support, this is disabled by
default.

Acked-by: Simon Glass <sjg@chromium.org>
Acked-by: Pantelis Antoniou <pantelis.antoniou@konsulko.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
diff --git a/test/overlay/Kconfig b/test/overlay/Kconfig
new file mode 100644
index 0000000..13c8542
--- /dev/null
+++ b/test/overlay/Kconfig
@@ -0,0 +1,11 @@
+config UT_OVERLAY
+	bool "Enable Device Tree Overlays Unit Tests"
+	depends on OF_LIBFDT_OVERLAY
+	depends on UNIT_TEST
+	help
+	  This enables the 'ut overlay' command which runs a series of unit
+	  tests on the fdt overlay code.
+	  If all is well then all tests pass although there will be a few
+	  messages printed along the way.
+	  Be warned that it requires an out-of-tree dtc compiler with patches
+	  to support the DT overlays, otherwise it will fail.