Move cross toolchain library install code out of setup_board to a
separate script, and have setup_board use it.
The install_toolchain script will be used by the builders to set up
a separate sysroot (distinct from the /build/<board> root), as part of
the Simplified Chrome Workflow (goto/simple-chrome).
BUG=chromium-os:36299
TEST=locally, trybots.
Change-Id: I88c355f1798da71ead9370a82365304dbf311504
Reviewed-on: https://gerrit.chromium.org/gerrit/38156
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Commit-Ready: Ryan Cui <rcui@chromium.org>
Tested-by: Ryan Cui <rcui@chromium.org>
