kernel: Turn on time-stamping (-N) for wget when getting kernel sources

wget downloads new linux-*.sign files and backs up the
existing files as file.1, file.2, etc. This causes false
positives during 'git status' or 'make check'
i.e.
* linux-2.6.38.3.tar.bz2.sign.1
* linux-3.2.7.tar.sign.1
Adding -N causes wget to smartly overwrite as required.