Rework building Pi images

First off, many thanks to @gwolf for having spent so much time providing support for Pi devices by building and publishing Pi images.

There have been on and off discussions about moving the build to some place on the official Debian infrastructure, and I'd like to help with that. Last time images were built automatically was early in 2024, so I'd suggest biting the bullet and looking into migrating now, as the current infrastructure no longer works.

Additionally, I'm not convinced it makes sense to ship per-device images anymore. The distinction should really be about the architecture instead: an arm64 image happily runs on Pi 3 and Pi 4, and once we've verified Pi 5 support, that should support all three of them. (I'm skipping details about CM4 support and the CMA thing in this executive summary, we could probably just do what's needed for it on that image, and let the reconfiguration that occurs during the first boot levels things out when running on the actual target.)