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/MPC837XEMDS_defconfig b/configs/MPC837XEMDS_defconfig
index 9ddddde..bf5e1b3 100644
--- a/configs/MPC837XEMDS_defconfig
+++ b/configs/MPC837XEMDS_defconfig
@@ -3,3 +3,4 @@
 CONFIG_TARGET_MPC837XEMDS=y
 # CONFIG_CMD_SETEXPR is not set
 CONFIG_SYS_NS16550=y
+CONFIG_OF_LIBFDT=y