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/O3DNT_defconfig b/configs/O3DNT_defconfig
index 8411294..5f2f0c1 100644
--- a/configs/O3DNT_defconfig
+++ b/configs/O3DNT_defconfig
@@ -2,3 +2,4 @@
 CONFIG_MPC5xxx=y
 CONFIG_TARGET_O3DNT=y
 # CONFIG_CMD_SETEXPR is not set
+CONFIG_OF_LIBFDT=y