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/iocon_defconfig b/configs/iocon_defconfig
index a6532d1..dac950e 100644
--- a/configs/iocon_defconfig
+++ b/configs/iocon_defconfig
@@ -7,3 +7,4 @@
 # CONFIG_CMD_ELF is not set
 # CONFIG_CMD_NFS is not set
 CONFIG_SYS_NS16550=y
+CONFIG_OF_LIBFDT=y