feat(coreos-base/coreos): Add ethtool to the base image.

ethtool has long since replaced the old mii-tool as the proper way to
get information on physical ethernet links but we haven't been shipping
it. At some point we should prune some of the old crufty tools like
mii-tool from sys-apps/net-tools and similar.
Merge pull request #500 from marineam/ethtool

feat(coreos-base/coreos): Add ethtool to the base image.