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/icon_defconfig b/configs/icon_defconfig
index 36fd884..ede63a7 100644
--- a/configs/icon_defconfig
+++ b/configs/icon_defconfig
@@ -2,3 +2,4 @@
 CONFIG_4xx=y
 CONFIG_TARGET_ICON=y
 CONFIG_SYS_NS16550=y
+CONFIG_OF_LIBFDT=y