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_aemv8a_juno_defconfig b/configs/vexpress_aemv8a_juno_defconfig
index bc3e05a..07036cc 100644
--- a/configs/vexpress_aemv8a_juno_defconfig
+++ b/configs/vexpress_aemv8a_juno_defconfig
@@ -16,3 +16,4 @@
 # CONFIG_CMD_NFS is not set
 # CONFIG_CMD_MISC is not set
 CONFIG_DM=y
+CONFIG_OF_LIBFDT=y