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/igep0020_defconfig b/configs/igep0020_defconfig
index 805a224..2731e94 100644
--- a/configs/igep0020_defconfig
+++ b/configs/igep0020_defconfig
@@ -9,3 +9,4 @@
 # CONFIG_CMD_SETEXPR is not set
 CONFIG_NET_RANDOM_ETHADDR=y
 CONFIG_SYS_NS16550=y
+CONFIG_OF_LIBFDT=y