netperf: upgrade to gentoo 2.7.0-r2

Upgrade netperf to latest gentoo version of netperf.

CQ-DEPEND=CL:889502
BUG=chromium:806326
TEST=cd /mnt/host/source/src/third_party/portage-stable/net-analyzer/netperf
     <cherry-pick from https://chromium-review.googlesource.com/877415>
     # assumes test/dev image already deployed on $B1/$B2
     cros deploy $B1 net-analyzer/netperf
     cros deploy $B2 net-analyzer/netperf

     netserver -V and netperf -V on respective machines should report:
     Netperf version 2.7.0

     (beware of /usr/local/bin/netperf vs /usr/bin/netperf)

     On $B1: "netserver" and "iptables -I INPUT 1 -j ACCEPT"
     On $B2: NETPERF=/usr/bin/netperf
        $NETPERF -H 192.168.xx.yy -t TCP_MAERTS -B "RX Mbps" -- -s 256K -S 256K

Change-Id: Ifd2b20ec6c2f15c3897c7db6ad89dc05a6b8d71f
Reviewed-on: https://chromium-review.googlesource.com/877415
Commit-Ready: Grant Grundler <grundler@chromium.org>
Tested-by: Grant Grundler <grundler@chromium.org>
Reviewed-by: Grant Grundler <grundler@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
(cherry picked from commit 0731367f0ffdd927ad61a004206e137655a2c75b)
Reviewed-on: https://chromium-review.googlesource.com/900169
Commit-Queue: Grant Grundler <grundler@chromium.org>
12 files changed