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/sbc8641d_defconfig b/configs/sbc8641d_defconfig
index a37b18e..6b19fc7 100644
--- a/configs/sbc8641d_defconfig
+++ b/configs/sbc8641d_defconfig
@@ -3,3 +3,4 @@
 CONFIG_TARGET_SBC8641D=y
 # CONFIG_CMD_SETEXPR is not set
 CONFIG_SYS_NS16550=y
+CONFIG_OF_LIBFDT=y