brltty service: wait for devices to settle

Otherwise it starts way too early, only to fail and having to restart
until devices are available.  It is less wasteful to simply wait until
there's a reasonable chance of success.  This is consistent with
upstream.

nixos/modules/services/hardware/brltty.nix

@@ -39,6 +39,8 @@ in {
         ProtectSystem = "full";
         SystemCallArchitectures = "native";
       };
+      wants = [ "systemd-udev-settle.service" ];
+      after = [ "local-fs.target" "systemd-udev-settle.service" ];
       before = [ "sysinit.target" ];
       wantedBy = [ "sysinit.target" ];
     };