overrideDerivation: fix meta handling (closes NixOS#7425)

stdenv.mkDerivation already implements the complicated addPassthru logic and
calls derivation for us, in particular filtering out "meta". Using it ensures
the invariant that
mkDerivation{<a>}.overrideDerivation{<b>} == mkDerivation{<a and b>}
Also, we give meta/passthru to the function, so that calls of the form
overrideDerivation(drv: meta = drv.meta // ...) work. Finally, we use
isFunction and recursiveMerge so that overrideDerivation { meta.x = y; }
works as well.

lib/customisation.nix

@@ -35,21 +35,9 @@ rec {
   */
   overrideDerivation = drv: f:
     let
-      newDrv = derivation (drv.drvAttrs // (f drv));
-    in addPassthru newDrv (
-      { meta = drv.meta or {};
-        passthru = if drv ? passthru then drv.passthru else {};
-      }
-      //
-      (drv.passthru or {})
-      //
-      (if (drv ? crossDrv && drv ? nativeDrv)
-       then {
-         crossDrv = overrideDerivation drv.crossDrv f;
-         nativeDrv = overrideDerivation drv.nativeDrv f;
-       }
-       else { }));
-
+      oldDrv = drv.drvAttrs // { meta = drv.meta or {}; passthru = drv.passthru or {}; crossAttrs = drv.crossAttrs or {}; };
+      newDrv = if builtins.isFunction f then f oldDrv else f;
+    in drv.stdenv.mkDerivation (lib.recursiveUpdate oldDrv newDrv);
 
   makeOverridable = f: origArgs:
     let

pkgs/stdenv/generic/default.nix

@@ -93,6 +93,7 @@ let
     , crossConfig ? null
     , meta ? {}
     , passthru ? {}
+    , crossAttrs ? {}
     , pos ? null # position used in error messages and for meta.position
     , ... } @ attrs:
     let
@@ -185,7 +186,7 @@ let
         meta = meta // (if pos' != null then {
           position = pos'.file + ":" + toString pos'.line;
         } else {});
-        inherit passthru;
+        inherit passthru crossAttrs;
       } //
       # Pass through extra attributes that are not inputs, but
       # should be made available to Nix expressions using the