boot: fdt: Move usable variable below updated comment

Move the variable below comment which explains what the variable means.
Update the comment. No functional change.

Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Suggested-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Marek Vasut <marek.vasut+renesas@mailbox.org>