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/O2DNT2_RAMBOOT_defconfig b/configs/O2DNT2_RAMBOOT_defconfig
index ca54ca5..cfd224c 100644
--- a/configs/O2DNT2_RAMBOOT_defconfig
+++ b/configs/O2DNT2_RAMBOOT_defconfig
@@ -6,3 +6,4 @@
 CONFIG_AUTOBOOT_PROMPT="Autobooting in %d seconds, press password to stop\n"
 CONFIG_AUTOBOOT_STOP_STR="++++++++++"
 # CONFIG_CMD_SETEXPR is not set
+CONFIG_OF_LIBFDT=y