nixos: tmp on tmpfs option

/tmp cleaning is done by systemd rather than stage-2-init
enableEmergencyMode moved from systemd to seperate module
new option to mount tmp on tmpfs
new option to enable additional units shipped with systemd

nixos/modules/system/boot/emergency-mode.nix

@@ -0,0 +1,37 @@
+{ config, lib, ... }:
+
+with lib;
+
+{
+
+  ###### interface
+
+  options = {
+
+    systemd.enableEmergencyMode = mkOption {
+      default = true;
+      type = types.bool;
+      description = ''
+        Whether to enable emergency mode, which is an
+        <command>sulogin</command> shell started on the console if
+        mounting a filesystem fails.  Since some machines (like EC2
+        instances) have no console of any kind, emergency mode doesn't
+        make sense, and it's better to continue with the boot insofar
+        as possible.
+      '';
+    };
+
+  };
+
+  ###### implementation
+
+  config = {
+
+    systemd.additionalUpstreamSystemUnits = optionals
+      config.systemd.enableEmergencyMode [
+        "emergency.target" "emergency.service"
+      ];
+
+  };
+
+}