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/a3m071_defconfig b/configs/a3m071_defconfig
index 3a6188e..6707e61 100644
--- a/configs/a3m071_defconfig
+++ b/configs/a3m071_defconfig
@@ -4,3 +4,4 @@
 CONFIG_SPL=y
 # CONFIG_CMD_SETEXPR is not set
 CONFIG_LIB_RAND=y
+CONFIG_OF_LIBFDT=y