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/MPC8540ADS_defconfig b/configs/MPC8540ADS_defconfig
index 1239ba8..f6906f7 100644
--- a/configs/MPC8540ADS_defconfig
+++ b/configs/MPC8540ADS_defconfig
@@ -2,3 +2,4 @@
 CONFIG_MPC85xx=y
 CONFIG_TARGET_MPC8540ADS=y
 CONFIG_SYS_NS16550=y
+CONFIG_OF_LIBFDT=y