nixos top-level: Add 'dtbs' symlink when kernel uses device trees

Currently e.g. extlinux-conf-builder.sh uses
`readlink -m "$toplevel/kernel/../dtbs"` to figure out the directory.
That is obscenely ugly.

nixos/modules/system/activation/top-level.nix

@@ -45,6 +45,9 @@ let
 
         ln -s ${kernelPath} $out/kernel
         ln -s ${config.system.modulesTree} $out/kernel-modules
+        ${optionalString (pkgs.stdenv.platform.kernelDTB or false) ''
+          ln -s ${config.boot.kernelPackages.kernel}/dtbs $out/dtbs
+        ''}
 
         echo -n "$kernelParams" > $out/kernel-params