dev: Add a version of EtherTap which uses the tap driver.

dev: Add a version of EtherTap which uses the tap driver.

The object is called EtherTap (as opposed to EtherTapStub, what the former
EtherTap was renamed to), and its existance is gated on the linux/if_tun.h
header file existing. That's probably overly strict, but it will hopefully
be minimally likely to break the build for other systems.

Change-Id: Ie03507fadf0d843a4d4d52f283c44a416c6f2a74
Reviewed-on: https://gem5-review.googlesource.com/3646
Reviewed-by: Nathan Binkert <nate@binkert.org>
Maintainer: Nathan Binkert <nate@binkert.org>