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/MPC8548CDS_36BIT_defconfig b/configs/MPC8548CDS_36BIT_defconfig
index 5e635e0..a880825 100644
--- a/configs/MPC8548CDS_36BIT_defconfig
+++ b/configs/MPC8548CDS_36BIT_defconfig
@@ -5,3 +5,4 @@
 CONFIG_NETDEVICES=y
 CONFIG_E1000=y
 CONFIG_SYS_NS16550=y
+CONFIG_OF_LIBFDT=y