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/MPC8560ADS_defconfig b/configs/MPC8560ADS_defconfig
index aa84d28..671f805 100644
--- a/configs/MPC8560ADS_defconfig
+++ b/configs/MPC8560ADS_defconfig
@@ -1,3 +1,4 @@
 CONFIG_PPC=y
 CONFIG_MPC85xx=y
 CONFIG_TARGET_MPC8560ADS=y
+CONFIG_OF_LIBFDT=y