feat: enable proper cross-compilation (x86_64 → aarch64) for all board modules

[gortium]

Problem

When cross-compiling (x86_64 to aarch64) with nixpkgs.buildPlatform = "x86_64-linux", all board modules source kernel and firmware from:

nixos-raspberrypi.packages.${pkgs.stdenv.hostPlatform.system}.linuxPackages_rpi5

During cross-compilation, hostPlatform resolves to "aarch64-linux", which looks up packages from mkRpiPkgs:

import nixpkgs { system = "aarch64-linux"; ... }

This is always a native aarch64 import, forcing QEMU user emulation for all kernel/firmware builds.

Solution

1. modules/nixpkgs-rpi.nix — use localSystem + crossSystem instead of system alone, so the re-imported pkgs.rpi respects cross-compilation when buildPlatform != hostPlatform.

2. All board modules — use pkgs.rpi.linuxPackages_rpiX / pkgs.rpi.raspberrypifw instead of nixos-raspberrypi.packages.${system}.linuxPackages_rpiX / .raspberrypifw. The module-level pkgs.rpi is now properly cross-compiled by fix #1.

This also aligns the board modules with the project architecture: pkgs.rpi (via nixpkgs-rpi) is the module-level internal API, while nixos-raspberrypi.packages is the flake-level public API for external consumers.

Files changed

• modules/nixpkgs-rpi.nix
• modules/raspberry-pi-5/default.nix
• modules/raspberry-pi-4.nix
• modules/raspberry-pi-3.nix
• modules/raspberry-pi-02.nix

Impact

• Cross-compilation works without QEMU: kernel, firmware, bootloader compile with the aarch64-unknown-linux-gnu toolchain
• No change for native aarch64 builds — identical results
• Tested on real CM5 hardware (uConsole) with cross-compiled SD image
• Related: PR Add cross-compilation for installer images (small, targeted change) #192 and PR Cross-compilation support, broken into small incremental commits #163 (neither addresses the board module package source)