fdt: Allow libfdt to be used in SPL

Add an option to enable libfdt in SPL. This can be useful when decoding
FIT files in SPL.

We need to make sure this option is not enabled in SPL by this change.
Also this option needs to be enabled in host builds. Si add a new
IMAGE_USE_LIBFDT #define which can be used in files that are built on the
host but must also build for U-Boot and SPL.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/lib/Kconfig b/lib/Kconfig
index d22a8e4..078ef5c 100644
--- a/lib/Kconfig
+++ b/lib/Kconfig
@@ -138,6 +138,16 @@
 	  particular compatible nodes. The library operates on a flattened
 	  version of the device tree.
 
+config SPL_OF_LIBFDT
+	bool "Enable the FDT library for SPL"
+	default y if SPL_OF_CONTROL
+	help
+	  This enables the FDT library (libfdt). It provides functions for
+	  accessing binary device tree images in memory, such as adding and
+	  removing notes and properties, scanning through the tree and finding
+	  particular compatible nodes. The library operates on a flattened
+	  version of the device tree.
+
 source lib/efi/Kconfig
 
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