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>

nixos/lib/from-env.nix

@@ -0,0 +1,4 @@
+# TODO: remove this file. There is lib.maybeEnv now
+name: default:
+let value = builtins.getEnv name; in
+if value == "" then default else value