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/apx4devkit_defconfig b/configs/apx4devkit_defconfig
index 5727929..e69fce2 100644
--- a/configs/apx4devkit_defconfig
+++ b/configs/apx4devkit_defconfig
@@ -4,3 +4,4 @@
 # CONFIG_CMD_IMLS is not set
 # CONFIG_CMD_FLASH is not set
 # CONFIG_CMD_SETEXPR is not set
+CONFIG_OF_LIBFDT=y