buildLinux/linuxManualConfig: Allow .override

Regression introduced by da36847d

.

The kernelPackages NixOS module option now uses .override to add kernel
patches defined in boot.kernelPatches into the kernel within the
kernelPackages set.

For generic kernels, we already have an .override which comes with
callPackage, but pkgs.linuxManualConfig is a function where we want to
override the attributes passed to it.

The callPackage to manual-config.nix only allows us to override its
internal attributes but not the attributes of the function returned.

Simplified it looks like this:

                                      .----- .override provided by
                                    __|__    callPackage
                                   |     |
(callPackage .../manual-config.nix { ... }) { ... }
                                            |_____|
                                               |
                                  the one -----'
                                  we actually want to override

Signed-off-by: aszlig <aszlig@redmoonstudios.org>