shill: connect Ethernet to PPPoEService

Add a new property (Ethernet.PPPoE) to Ethernet devices which allows
them to be put into PPPoE mode.  In PPPoE mode, a PPPoEService is
constructed and used in place of an EthernetService.

BUG=chrome-os-partner:32173
TEST=FEATURES=test USE="vpn cellular pppoe" emerge-x86-generic shill

Change-Id: I9d3c1c9eb542b55b9069c5ceeb99203dfd59a422
Reviewed-on: https://chromium-review.googlesource.com/246590
Reviewed-by: Paul Stewart <pstew@chromium.org>
Trybot-Ready: Garret Kelly <gdk@chromium.org>
Commit-Queue: Garret Kelly <gdk@chromium.org>
Tested-by: Garret Kelly <gdk@chromium.org>
5 files changed