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/wtk_defconfig b/configs/wtk_defconfig
index 7d4f754..5069487 100644
--- a/configs/wtk_defconfig
+++ b/configs/wtk_defconfig
@@ -3,3 +3,4 @@
 CONFIG_TARGET_TQM823L=y
 CONFIG_SYS_EXTRA_OPTIONS="LCD,SHARP_LQ065T9DR51U"
 # CONFIG_CMD_SETEXPR is not set
+CONFIG_OF_LIBFDT=y