Severely outdated BTStack library

[Danappelxx]

The last time the btstack library was pulled was over two years ago. Since then there has been over 2600 commits to btstack. Naturally there are some API changes but from my local-scale comparisons they seem mostly compatible.

bluekitchen/btstack@ebd8f44...master

At the very least updating will bring in bug fixes. As for feature improvements, the current version in the firmware does not even enable adding descriptors to a gatt characteristic definition, a basic feature. I'm sure there are more but that's the one I was looking at for my own application.

I would appreciate if the library was updated, or given guidance on how to do it myself.