fix(setup_board): Remove unused option to accept restrictive licenses
fix(sdk_lib): Delete a pile of dead/pointless code.

For the most part this doesn't influence anything. The one exception is
the custom configuration for using curl is dropped, just rely on the
portage defaults. It appears curl was only used to work around a wget
issue with Google's internal SSL certificates. We care not. :)
refactor(update_chroot): Write make.conf.host_setup

There is no need for this to be in sdk_lib any more, remove what little
remains of make_conf_util.sh.
fix(update_chroot): Consolidate more portage configuration

To behave more like setup_board/build_packages update_chroot should
fully configure portage to make sure everything is accurate.

Now binhosts are defined in make.conf.host_setup so the static config in
coreos-overlays doesn't need to refer to version.txt. setup_board
already made this change in 7a43a07f.

Define path locations to reduce dependency between static configs in
coreos-overlays and the behavior of the scripts repo. Spreading
configuration across two repos makes everything harder to understand.
Eventually everything should either be defined in profiles in
coreos-overlays or minimal auto-generated config files here in scripts.
Merge pull request #251 from marineam/refactor

Refactor sdk setup