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/MPC8555CDS_legacy_defconfig b/configs/MPC8555CDS_legacy_defconfig
index eb6382c..bc069e1 100644
--- a/configs/MPC8555CDS_legacy_defconfig
+++ b/configs/MPC8555CDS_legacy_defconfig
@@ -3,3 +3,4 @@
 CONFIG_TARGET_MPC8555CDS=y
 CONFIG_SYS_EXTRA_OPTIONS="LEGACY"
 CONFIG_SYS_NS16550=y
+CONFIG_OF_LIBFDT=y