nixos/power-management: run postBootCommands in a systemd service instead of stage2-init.sh

This reduces our initrd script slightly, and we never made any clear
ordering guarantees about when these commands run anyway.
It also removes this as a blocker for nixos-init.

nixos/modules/config/power-management.nix

@@ -81,6 +81,21 @@ in
       serviceConfig.Type = "oneshot";
     };
 
+    systemd.services.post-boot = {
+      description = "Post-boot Actions";
+      # It's not well defined at what point in the bootup sequence this should run
+      # we should eventually just remove this.
+      after = [ "multi-user.target" ];
+      wantedBy = [ "multi-user.target" ];
+      serviceConfig = {
+        Type = "oneshot";
+        RemainAfterExit = true;
+      };
+      script = ''
+        ${cfg.powerUpCommands}
+      '';
+    };
+
     systemd.services.post-resume = {
       description = "Post-Resume Actions";
       # Pulled in by post-resume.service above

nixos/modules/system/activation/nixos-init.nix

@@ -58,10 +58,6 @@ in
           assertion = config.boot.postBootCommands == "";
           message = "nixos-init cannot be used with boot.postBootCommands";
         }
-        {
-          assertion = config.powerManagement.powerUpCommands == "";
-          message = "nixos-init cannot be used with powerManagement.powerUpCommands";
-        }
       ];
     })
   ];

nixos/modules/system/boot/stage-2.nix

@@ -30,7 +30,6 @@ let
       );
       postBootCommands = pkgs.writeText "local-cmds" ''
         ${config.boot.postBootCommands}
-        ${config.powerManagement.powerUpCommands}
       '';
     };
   };