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_defconfig b/configs/digsy_mtc_defconfig
index 913ba7e..8a134fd 100644
--- a/configs/digsy_mtc_defconfig
+++ b/configs/digsy_mtc_defconfig
@@ -5,3 +5,4 @@
 CONFIG_AUTOBOOT_PROMPT="autoboot in %d seconds\n"
 CONFIG_AUTOBOOT_DELAY_STR=" "
 # CONFIG_CMD_SETEXPR is not set
+CONFIG_OF_LIBFDT=y