add vivado buildBitstream derivation

vivado.nix

@@ -0,0 +1,37 @@
+# Install Vivado in /opt and add to /etc/nixos/configuration.nix:
+#  nix.sandboxPaths = ["/opt"];
+
+{ pkgs ? import <nixpkgs> {}}:
+let
+  vivadoEnv = pkgs.buildFHSUserEnv {
+    name = "vivado-env";
+    targetPkgs = pkgs: (
+      with pkgs; [
+        ncurses5
+        zlib
+        libuuid
+        xorg.libSM
+        xorg.libICE
+        xorg.libXrender
+        xorg.libX11
+        xorg.libXext
+        xorg.libXtst
+        xorg.libXi
+      ]
+    );
+  };
+in
+  {
+    buildBitstream = { name, src, vivadoPath ? "/opt/Xilinx/Vivado/2018.3" }:
+      pkgs.stdenv.mkDerivation {
+        inherit name src;
+        phases = [ "buildPhase" ];
+        buildPhase =
+          ''
+          cp --no-preserve=mode,ownership -R $src/* .
+          ${vivadoEnv}/bin/vivado-env -c "source ${vivadoPath}/settings64.sh && vivado -mode batch -source top.tcl"
+          mkdir $out
+          cp *.dcp *.rpt *.bit $out
+          '';
+  	  };
+  }