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.
1) For newer pvops kernels we can use kernel.org sign files that are signed
with a new (uncompromised, at least not known to be compromised yet) key
2) For older kernel, we need to use hash, as the kernel.org decided not to
release update signature with a new key.
In any case, use hash-based verification additionally, try to minimize trust put
into kernel.org people...
