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/ac14xx_defconfig b/configs/ac14xx_defconfig
index 608b56a..21cb0b8 100644
--- a/configs/ac14xx_defconfig
+++ b/configs/ac14xx_defconfig
@@ -3,3 +3,4 @@
 CONFIG_TARGET_AC14XX=y
 CONFIG_SYS_PROMPT="ac14xx> "
 # CONFIG_CMD_SETEXPR is not set
+CONFIG_OF_LIBFDT=y