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/MPC8323ERDB_defconfig b/configs/MPC8323ERDB_defconfig
index 337a0dd..003c592 100644
--- a/configs/MPC8323ERDB_defconfig
+++ b/configs/MPC8323ERDB_defconfig
@@ -3,3 +3,4 @@
 CONFIG_TARGET_MPC8323ERDB=y
 # CONFIG_CMD_SETEXPR is not set
 CONFIG_SYS_NS16550=y
+CONFIG_OF_LIBFDT=y