Adopt Yocto/OpenEmbedded as the Pi OS build system

[jkaczman]

The current Pi image build starts from a prebuilt Raspberry Pi OS Bookworm .img, mounts it, mutates it in place, and then ships that modified image as the result. We need to replace that with a Yocto/OpenEmbedded-based OS build, where we set the system through pinned layers / machine configuration / image recipes / boot assets, and then generate the final image from that build definition rather than trying to mess with a pre-made disk image (Raspberry Pi OS Lite) afterward.