cleanup(setup_board): Remove /etc/make.conf.board

This file mostly just defined BINHOSTs but I'd like to move that
completely into coreos-overlay as the SDK BINHOSTs have always been. It
also sourced src/overlays/overlay-amd64-generic/make.conf but I want to
move the few things it defined into coreos-overlay as well.

The one interesting thing this file did was to optionally define
ACCEPT_LICENSE but that can go into /etc/make.conf.board_setup instead.

Overall this takes a chunk out of the make.conf spaghetti. :)