nixos: move bcache udev rule in a new bcache.nix module

nixos/modules/module-list.nix

···
   ./system/boot/tmp.nix
   ./system/etc/etc.nix
   ./system/upstart/upstart.nix
+  ./tasks/bcache.nix
   ./tasks/cpu-freq.nix
   ./tasks/encrypted-devices.nix
   ./tasks/filesystems.nix

nixos/modules/system/boot/stage-1.nix

···
       cp -v ${udev}/lib/udev/rules.d/60-persistent-storage.rules $out/
       cp -v ${udev}/lib/udev/rules.d/80-drivers.rules $out/
       cp -v ${pkgs.lvm2}/lib/udev/rules.d/*.rules $out/
-      cp -v ${pkgs.bcache-tools}/lib/udev/rules.d/*.rules $out/
       ${config.boot.initrd.extraUdevRulesCommands}
 
       for i in $out/*.rules; do

nixos/modules/tasks/bcache.nix

···
+{ config, pkgs, ... }:
+
+{
+
+  environment.systemPackages = [ pkgs.bcache-tools ];
+
+  boot.initrd.extraUdevRulesCommands = ''
+    cp -v ${pkgs.bcache-tools}/lib/udev/rules.d/*.rules $out/
+  ''; 
+
+}