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/digsy_mtc_RAMBOOT_defconfig b/configs/digsy_mtc_RAMBOOT_defconfig
index 17ac961..9055029 100644
--- a/configs/digsy_mtc_RAMBOOT_defconfig
+++ b/configs/digsy_mtc_RAMBOOT_defconfig
@@ -7,3 +7,4 @@
 CONFIG_AUTOBOOT_DELAY_STR="d"
 CONFIG_AUTOBOOT_STOP_STR=" "
 # CONFIG_CMD_SETEXPR is not set
+CONFIG_OF_LIBFDT=y