image: Drop IMAGE_ENABLE_OF_LIBFDT

Add a host Kconfig for OF_LIBFDT. With this we can use
CONFIG_IS_ENABLED(OF_LIBFDT) directly in the tools build, so drop the
unnecessary indirection.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
diff --git a/tools/Kconfig b/tools/Kconfig
index 6ffc2c0..747d221 100644
--- a/tools/Kconfig
+++ b/tools/Kconfig
@@ -50,6 +50,11 @@
 	help
 	  Enable MD5 support in the tools builds
 
+config TOOLS_OF_LIBFDT
+	def_bool y
+	help
+	  Enable libfdt support in the tools builds
+
 config TOOLS_SHA1
 	def_bool y
 	help