Move CONFIG_OF_LIBFDT to Kconfig

Move this option to Kconfig and tidy up existing boards.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/vexpress_ca5x2_defconfig b/configs/vexpress_ca5x2_defconfig
index ce5da91..b260401 100644
--- a/configs/vexpress_ca5x2_defconfig
+++ b/configs/vexpress_ca5x2_defconfig
@@ -14,3 +14,4 @@
 # CONFIG_CMD_SETEXPR is not set
 # CONFIG_CMD_NFS is not set
 # CONFIG_CMD_MISC is not set
+CONFIG_OF_LIBFDT=y