Add a nixos config for the raspberry pi 3b.

nix/raspberry_pi/flake.nix

@@ -0,0 +1,46 @@
+# nix build .#nixosConfigurations.rpi3.config.system.build.sdImage
+{
+  description = "Minimal NixOS Raspberry Pi 3B Flake";
+
+  inputs = {
+    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
+  };
+
+  outputs =
+    { self, nixpkgs, ... }@inputs:
+    {
+      nixosConfigurations.rpi3 = nixpkgs.lib.nixosSystem {
+        system = "aarch64-linux";
+        modules = [
+          # Include the official SD card image builder module
+          "${nixpkgs}/nixos/modules/installer/sd-card/sd-image-aarch64.nix"
+
+          ({ lib, pkgs, ... }: {
+            # System basic configuration
+            system.stateVersion = "24.11";
+            networking.hostName = "nixos-pi3";
+
+            # Enable necessary hardware firmware
+            hardware.enableRedistributableFirmware = true;
+
+            # Create a default user so you can log in
+            users.users.root.initialHashedPassword = ""; # Or set a password/SSH key
+            users.users.nixos = {
+              isNormalUser = true;
+              extraGroups = [ "wheel" ]; # Enable ‘sudo’
+              initialPassword = "changeme";
+            };
+
+            # Enable SSH for remote access on first boot
+            services.openssh = {
+              enable = true;
+              settings.PermitRootLogin = "yes"; # Change or use keys for security
+            };
+
+            # Expand root partition automatically on first boot
+            # services.growpart.enable = true;
+          })
+        ];
+      };
+    };
+}