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/sniper_defconfig b/configs/sniper_defconfig
index 0335795..370c5fd 100644
--- a/configs/sniper_defconfig
+++ b/configs/sniper_defconfig
@@ -12,3 +12,4 @@
 CONFIG_SYS_NS16550=y
 CONFIG_USB=y
 CONFIG_USB_MUSB_GADGET=y
+CONFIG_OF_LIBFDT=y