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/riotboard_defconfig b/configs/riotboard_defconfig
index 3cf82c3..35b96a3 100644
--- a/configs/riotboard_defconfig
+++ b/configs/riotboard_defconfig
@@ -9,3 +9,4 @@
 CONFIG_SPI_FLASH=y
 CONFIG_SPI_FLASH_SST=y
 CONFIG_DM_THERMAL=y
+CONFIG_OF_LIBFDT=y