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/axm_defconfig b/configs/axm_defconfig
index 25e4570..65c13eb 100644
--- a/configs/axm_defconfig
+++ b/configs/axm_defconfig
@@ -13,3 +13,4 @@
 CONFIG_SPI_FLASH=y
 CONFIG_SPI_FLASH_STMICRO=y
 CONFIG_USE_TINY_PRINTF=y
+CONFIG_OF_LIBFDT=y