iio-sensor-proxy: init at 2.2 and nixos module

This PR adds support for ```iio-sensor-proxy``` used by GNOME v3 and
others for reading data from the accelerometer, gps, compass and similar sensors
built into some relatively recent laptops.

Additionally, there is a NixOS module exposed via hardware.sensor.iio
for enabling services, udev rules and dbus services.

nixos/modules/hardware/sensor/iio.nix

@@ -0,0 +1,30 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+{
+  ###### interface
+
+  options = {
+    hardware.sensor.iio = {
+      enable = mkOption {
+        description = "Enable this option to support IIO sensors.";
+        type = types.bool;
+        default = false;
+      };
+    };
+  };
+
+  ###### implementation
+
+  config = mkIf config.hardware.sensor.iio.enable {
+
+    boot.initrd.availableKernelModules = [ "hid-sensor-hub" ];
+
+    environment.systemPackages = with pkgs; [ iio-sensor-proxy ];
+
+    services.dbus.packages = with pkgs; [ iio-sensor-proxy ];
+    services.udev.packages = with pkgs; [ iio-sensor-proxy ];
+    systemd.packages = with pkgs; [ iio-sensor-proxy ];
+  };
+}