NixOS/amazonImageZfs: init

Introduce an AWS EC2 AMI which supports aarch64 and x86_64 with a ZFS
root.

This uses `make-zfs-image` which implies two EBS volumes are needed
inside EC2, one for boot, one for root. It should not matter which
is identified `xvda` and which is `xvdb`, though I have always
uploaded `boot` as `xvda`.

(cherry picked from commit bd38b059eae05871579b2dfd51cd41d058b6a1ec)

nixos/modules/virtualisation/amazon-image.nix

@@ -33,17 +33,23 @@ in
 
     boot.growPartition = cfg.hvm;
 
-    fileSystems."/" = {
+    fileSystems."/" = mkIf (!cfg.zfs.enable) {
       device = "/dev/disk/by-label/nixos";
       fsType = "ext4";
       autoResize = true;
     };
 
-    fileSystems."/boot" = mkIf cfg.efi {
+    fileSystems."/boot" = mkIf (cfg.efi || cfg.zfs.enable) {
+      # The ZFS image uses a partition labeled ESP whether or not we're
+      # booting with EFI.
       device = "/dev/disk/by-label/ESP";
       fsType = "vfat";
     };
 
+    services.zfs.expandOnBoot = mkIf cfg.zfs.enable "all";
+
+    boot.zfs.devNodes = mkIf cfg.zfs.enable "/dev/";
+
     boot.extraModulePackages = [
       config.boot.kernelPackages.ena
     ];