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/warp_defconfig b/configs/warp_defconfig
index f112d33..959e29c 100644
--- a/configs/warp_defconfig
+++ b/configs/warp_defconfig
@@ -8,3 +8,4 @@
 # CONFIG_CMD_SETEXPR is not set
 # CONFIG_CMD_NET is not set
 # CONFIG_CMD_NFS is not set
+CONFIG_OF_LIBFDT=y