libfdt: Add overlay application function

The device tree overlays are a good way to deal with user-modifyable
boards or boards with some kind of an expansion mechanism where we can
easily plug new board in (like the BBB, the Raspberry Pi or the CHIP).

Add a new function to merge overlays with a base device tree.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
diff --git a/lib/Kconfig b/lib/Kconfig
index 02ca405..16ff01a 100644
--- a/lib/Kconfig
+++ b/lib/Kconfig
@@ -139,6 +139,11 @@
 	  particular compatible nodes. The library operates on a flattened
 	  version of the device tree.
 
+config OF_LIBFDT_OVERLAY
+	bool "Enable the FDT library overlay support"
+	help
+	  This enables the FDT library (libfdt) overlay support.
+
 config SPL_OF_LIBFDT
 	bool "Enable the FDT library for SPL"
 	default y if SPL_OF_CONTROL