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/mpc5121ads_rev2_defconfig b/configs/mpc5121ads_rev2_defconfig
index 3562353..b26f6dd 100644
--- a/configs/mpc5121ads_rev2_defconfig
+++ b/configs/mpc5121ads_rev2_defconfig
@@ -3,3 +3,4 @@
 CONFIG_TARGET_MPC5121ADS=y
 CONFIG_SYS_EXTRA_OPTIONS="MPC5121ADS_REV2"
 # CONFIG_CMD_SETEXPR is not set
+CONFIG_OF_LIBFDT=y