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/mx6qpsabreauto_defconfig b/configs/mx6qpsabreauto_defconfig
index c44cd8f..0877c41 100644
--- a/configs/mx6qpsabreauto_defconfig
+++ b/configs/mx6qpsabreauto_defconfig
@@ -5,3 +5,4 @@
 CONFIG_CMD_GPIO=y
 CONFIG_SPI_FLASH=y
 CONFIG_SPI_FLASH_STMICRO=y
+CONFIG_OF_LIBFDT=y