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/tmp.nix

@@ -0,0 +1,39 @@
+{ config, lib, ... }:
+
+with lib;
+
+{
+
+  ###### interface
+
+  options = {
+
+    boot.cleanTmpDir = mkOption {
+      type = types.bool;
+      default = false;
+      description = ''
+        Whether to delete all files in <filename>/tmp</filename> during boot.
+      '';
+    };
+
+    boot.tmpOnTmpfs = mkOption {
+      type = types.bool;
+      default = false;
+      description = ''
+         Whether to mount a tmpfs on <filename>/tmp</filename> during boot.
+      '';
+    };
+
+  };
+
+  ###### implementation
+
+  config = {
+
+    systemd.additionalUpstreamSystemUnits = optional config.boot.tmpOnTmpfs "tmp.mount";
+
+    systemd.tmpfiles.rules = optional config.boot.cleanTmpDir "D! /tmp 1777 root root";
+
+  };
+
+}