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/dlvision-10g_defconfig b/configs/dlvision-10g_defconfig
index fe15a4a..9dbcbb4 100644
--- a/configs/dlvision-10g_defconfig
+++ b/configs/dlvision-10g_defconfig
@@ -6,3 +6,4 @@
 # CONFIG_CMD_ELF is not set
 # CONFIG_CMD_NFS is not set
 CONFIG_SYS_NS16550=y
+CONFIG_OF_LIBFDT=y