SW: use Debian kernel packages instead of our custom Grml ones

Maintaining and keeping our linux-image-i386-grml +
linux-image-amd64-grml packages requires manpower we currently
don't really have. Instead let's see whether switching to the
kernel packages provided by Debian fits our needs. (This might
not be the case if we need different configuration options,
defaults, extra patches or the release cycle doesn't fit for
us, but it's worth a try, so let's find out.)

See https://github.com/grml/grml/issues/139