uboot: (firmwareOdroidC2/C4) don't invoke patch tool, use patches = [] instead

https://github.com/NixOS/nixpkgs/blob/master/pkgs/stdenv/generic/setup.sh#L948
this can do it nicely.

Signed-off-by: Anton Arapov <anton@deadbeef.mx>

pkgs/build-support/dhall/package-to-nix.nix

@@ -0,0 +1,36 @@
+
+# `dhallPackageToNix` is a utility function to take a Nixpkgs Dhall package
+# (created with a function like `dhallPackages.buildDhallDirectoryPackage`)
+# and read it in as a Nix expression.
+#
+# This function is similar to `dhallToNix`, but takes a Nixpkgs Dhall package
+# as input instead of raw Dhall code.
+#
+# Note that this uses "import from derivation" (IFD), meaning that Nix will
+# perform a build during the evaluation phase if you use this
+# `dhallPackageToNix` utility.  It is not possible to use `dhallPackageToNix`
+# in Nixpkgs, since the Nixpkgs Hydra doesn't allow IFD.
+
+{ stdenv, dhall-nix }:
+
+dhallPackage:
+  let
+    drv = stdenv.mkDerivation {
+      name = "dhall-compiled-package.nix";
+
+      buildCommand = ''
+        # Dhall requires that the cache is writable, even if it is never written to.
+        # We copy the cache from the input package to the current directory and
+        # set the cache as writable.
+        cp -r "${dhallPackage}/.cache" ./
+        export XDG_CACHE_HOME=$PWD/.cache
+        chmod -R +w ./.cache
+
+        dhall-to-nix <<< "${dhallPackage}/binary.dhall" > $out
+      '';
+
+      nativeBuildInputs = [ dhall-nix ];
+    };
+
+  in
+    import drv