Add the ability to provide an old boards.txt to the boards generator

[dmlambo]

This change is to allow you to feed in an old boards.txt with hand-written additions at the top, separated from the automated content by "## Auto-generated content here", and also notes the order in which boards were previously added, and adds any new boards to the bottom of the file, which makes diffing much cleaner and easy to read.

Also changes the vid pid to 1eaf 0003, since that appears to be the vid/pid of the majority of boards.

Also adds gd32f10x to enabled USB devices, but the devices aren't enabled by this change.