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/platinum_picon_defconfig b/configs/platinum_picon_defconfig
index c4a82d7..b7e5d7c 100644
--- a/configs/platinum_picon_defconfig
+++ b/configs/platinum_picon_defconfig
@@ -8,3 +8,4 @@
 # CONFIG_CMD_FLASH is not set
 CONFIG_CMD_GPIO=y
 # CONFIG_CMD_SETEXPR is not set
+CONFIG_OF_LIBFDT=y