Add Xilinx xocl derivation

Need to move to raccoon to access "config" attribute.

pkgs/overlay.nix

@@ -42,5 +42,4 @@ final: prev:
   });
 
   prometheus-slurm-exporter = prev.callPackage ./slurm-exporter.nix { };
-  xilinx-xrt = prev.callPackage ./xilinx-xrt.nix { };
 }