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/nitrogen6s_defconfig b/configs/nitrogen6s_defconfig
index 9b44758..707fb58 100644
--- a/configs/nitrogen6s_defconfig
+++ b/configs/nitrogen6s_defconfig
@@ -7,3 +7,4 @@
 CONFIG_CMD_GPIO=y
 CONFIG_SPI_FLASH=y
 CONFIG_SPI_FLASH_SST=y
+CONFIG_OF_LIBFDT=y