Refactored some stuff & made it more CustoPiZer-ic

[foosel]

• Got rid of gitclone which really serves no purpose here and only adds a ton of unnecessary complexity
• Fixed permissions of the klipper checkout & venv (otherwise Klipper would complain about not being able to compile some native extension during runtime)
• Moved the virtual env into the klipper folder. That's something I would have done for OctoPrint itself if it weren't this deeply ingrained into the community now. Source with venv included is simply less littering than source and venv next to each other.
• Restructured /files to only contain what is needed, copy that over manually, and also merge the OctoPrint config instead of overwriting it. The reason being that THIS way you can reuse this with a later OctoPi base image that might have to add or change anything in the shipped config without having to remember to fix this here as well. Only the klipper specific config adjustments are included.
• Removed INCLUDE flags. The build is short and it doesn't make much sense to not include Klipper on an image specifically built to include Klipper. Less noise this way.
• And in the end some clean-up: Moved some lines around to keep logical units together more clearly, added some comments and fixed some overlooked refactoring errors (I did all the changes at once but then realized you might want to have them as individual commits to be able to better follow what I changed, and some stuff slipped through because I didn't test the build for the individual steps of the whole untangling).

All in all that reduced the actual build script by around a 100 lines and not only made it more readable but also better reusable and more in line with the idea of CustoPiZer.

Personally I'd also not have put the stuff into a src folder (see https://github.com/OctoPrint/OctoPi-UpToDate) because there really isn't that much there apart from scripts and files but YMMV.

Build was tested locally (docker run ...), flashed to a Pi on my testrig, booted and verified that the Klipper server is running and OctoPrint/the plugin able to connect to it.

Sidenote: I have my doubts about the image shrinking working as it should, I cannot imagine that the stuff added by this build really fills up the whole 2GB. I'll take another look on CustoPiZer/OctoPi at that ASAP.

[guysoft]

Cool, merged, I still need to figure out how to do the github actions. I am still not sure that CustoPiZer is the way to go here and not CustomPiOS. Which I already have up and building. I wanted to make this repo and put this comment, because if anyone else is reading this who is not @foosel or me it would help me if you voice your preference too.