add nixos module

base.nix

@@ -1,4 +1,4 @@
-{ config, pkgs, modulesPath, ... }: {
+{ config, pkgs, lib, modulesPath, ... }: {
   fileSystems = {
     "/boot" = {
       device = "/dev/disk/by-label/nixos-boot";
@@ -16,11 +16,50 @@
   system.build.rootFileSystem = pkgs.callPackage (modulesPath + "/../lib/make-ext4-fs.nix") {
     storePaths = with pkgs; [
       config.system.build.toplevel
-      busybox
     ];
     volumeLabel = "nixos-root";
+    populateImageCommands = ''
+      cp ${config.system.build.toplevel}/init ./files/init
+      touch ./files/firstboot
+    '';
   };
 
+  # from https://github.com/NixOS/nixpkgs/blob/e405f30513169feedb64b5c25e7b00242010af58/nixos/modules/installer/sd-card/sd-image.nix#L267
+  boot.postBootCommands = let
+      expandOnBoot = ''
+        # Figure out device names for the boot device and root filesystem.
+        rootPart=$(${pkgs.util-linux}/bin/findmnt -n -o SOURCE /)
+        bootDevice=$(lsblk -npo PKNAME $rootPart)
+        partNum=$(lsblk -npo MAJ:MIN $rootPart | ${pkgs.gawk}/bin/awk -F: '{print $2}')
+
+        # Resize the root partition and the filesystem to fit the disk
+        echo ",+," | sfdisk -N$partNum --no-reread $bootDevice
+        ${pkgs.parted}/bin/partprobe
+        ${pkgs.e2fsprogs}/bin/resize2fs $rootPart
+      '';
+      firstbootFile = "/firstboot";
+    in ''
+      # On the first boot do some maintenance tasks
+      if [ -f ${firstbootFile} ]; then
+        set -euo pipefail
+        set -x
+
+        ${expandOnBoot}
+
+        # TODO what does all this do?
+
+        # Register the contents of the initial Nix store
+        # ${config.nix.package.out}/bin/nix-store --load-db < ${firstbootFile}
+
+        # nixos-rebuild also requires a "system" profile and an /etc/NIXOS tag.
+        # touch /etc/NIXOS
+        # ${config.nix.package.out}/bin/nix-env -p /nix/var/nix/profiles/system --set /run/current-system
+
+        # Prevents this from running on later boots.
+        rm -f ${firstbootFile}
+      fi
+    '';
+
   # system.build.usbImage = pkgs.callPackage ({ stdenvNoCC }: stdenvNoCC.mkDerivation {
   #   name = "nixos-ps4-usbimage";