flake: create generator for aarch64 sdcard image

hosts/caramel/filesystem.nix

@@ -1,4 +1,4 @@
-{ config, ... }:
+{ config, lib, ... }:
 let
   bind = src: {
     depends = [ "/persist" ];
@@ -9,16 +9,31 @@ let
   };
 in {
   fileSystems = {
-    "/" = {
+    "/" = lib.mkForce {
       device = "rootfs";
       fsType = "tmpfs";
-      options = [ "defaults" "size=2G" "mode=755" ];
+      options = [ "defaults" "size=1G" "mode=755" ];
     };
 
-    "/nix" = {
+    # "/nix" = {
+    #   device = "overlayfs";
+    #   fsType = "overlay";
+    #   options = [
+    #     "lowerdir=/mnt/image/nix"
+    #     "upperdir=/persist/nix-overlay"
+    #     "workdir=/persist/.overlaytmp"
+    #   ];
+    #   noCheck = true;
+    #   depends = [ "/mnt/image" "/persist" ];
+    # };
+
+    "/nix" = (bind "/mnt/image/nix") // { depends = [ "/mnt/image" ]; };
+
+    "/mnt/image" = {
       device = "/dev/disk/by-label/NIXOS_SD";
       fsType = "ext4";
       options = [ "defaults" "noatime" ];
+      neededForBoot = true;
     };
 
     "/persist" = {
@@ -31,6 +46,6 @@ in {
     "/var/persist" = bind "/persist";
     "/var/lib/acme" = bind "/persist/acme";
     "/var/log/journal" = bind "/persist/journal";
-    "/boot" = (bind "/nix/persist/boot") // { depends = [ "/nix" ]; };
+    "/boot" = (bind "/mnt/image/boot") // { depends = [ "/mnt/image" ]; };
   };
 }