convert devices from overlay to module

this makes it possible for devices to stash things in
`config` (e.g. pathname to DTS) that can later be overwritten
(even by the end user)

pkgs/kernel/default.nix

@@ -6,7 +6,7 @@
 
  , config
  , src
- , extraPatchPhase ? "true"
+ , extraPatchPhase ? "echo"
 } :
 let writeConfig = name : config: writeText name
         (builtins.concatStringsSep