fdtdec: Add Kconfig symbol for tests

Runtime tests are provided as a test_fdtdec command implementation. Add
a Kconfig symbol that allows this command to be built so that the tests
can be used.

Signed-off-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/lib/Kconfig b/lib/Kconfig
index 8fe5d85..2120216 100644
--- a/lib/Kconfig
+++ b/lib/Kconfig
@@ -436,4 +436,8 @@
 source lib/efi_loader/Kconfig
 source lib/optee/Kconfig
 
+config TEST_FDTDEC
+	bool "enable fdtdec test"
+	depends on OF_LIBFDT
+
 endmenu