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/usbarmory_defconfig b/configs/usbarmory_defconfig
index c25d103..68f78b3 100644
--- a/configs/usbarmory_defconfig
+++ b/configs/usbarmory_defconfig
@@ -3,3 +3,4 @@
 CONFIG_TARGET_USBARMORY=y
 # CONFIG_CMD_IMLS is not set
 # CONFIG_CMD_SETEXPR is not set
+CONFIG_OF_LIBFDT=y