Moved the hardcoded postBuild hook from the builder to generic.nix:
Some old kernel (such as 2.6.15) did not yet support the unifdef target.
As a result, compiling them with the current Linux builder leads to a
failure.
Fixed by moving this hook as argument of the top-level function of
generic.nix. This allows some kernel nix codes to overrides its default
value.
svn path=/nixpkgs/trunk/; revision=27708
8268a39690