Make use of cros_setup_toolchains, take 2

Resubmit of If62b4f3973f02fd8e1deed35864c824a02ab0c22
This will be safe to land after
I2c4e21ec7e8c0c0cf58947e2b0a3a9edf7617a09
The breakage was a timing issue paired with people not always syncing
the complete tree. No changes to the CL are needed.

It is now used for:
- make_chroot (cros_sdk --bootstrap)
- update_chroot

setup_board is stripped of redundant code which was deprecated by this.

Also stripped is some usepkg logic in make_chroot, as that is now
exclusively source-only.

BUG=chromium-os:23032
TEST=trybot chromiumos-sdk

Change-Id: Ib888cf2886218622d9cfeebb17b9cd4462d06c89
Reviewed-on: https://gerrit.chromium.org/gerrit/22578
Tested-by: Zdenek Behan <zbehan@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: asharif <asharif@chromium.org>
Commit-Ready: Zdenek Behan <zbehan@chromium.org>
3 files changed