chromium/common: Allow to override vanilla source.

The main output of the Chromium source derivation includes a lot of
build-specific files that may be required for projects based on the
Chromium source tree.

So we're creating the link farm on postUnpack now and also unpack the
additional source archive(s) in this step.

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