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/mx6slevk_defconfig b/configs/mx6slevk_defconfig
index 5eca5e9..2d24622 100644
--- a/configs/mx6slevk_defconfig
+++ b/configs/mx6slevk_defconfig
@@ -10,3 +10,4 @@
 CONFIG_SPI_FLASH=y
 CONFIG_SPI_FLASH_STMICRO=y
 CONFIG_DM_THERMAL=y
+CONFIG_OF_LIBFDT=y