setup_board: fix --regen_config bug.
setup_board: fix --regen_config bug.

https://gerrit.chromium.org/gerrit/#/c/38156 introduced a bug where
cross_get_version was moved to install_toolchain but was still
used in setup_board.

setup_board now saves whatever toolchain version is currently configured
and appends that information to the newly generated board config.

BUG=chromium-os:36865
TEST=remote trybot. manual - test case of existence/nonexistence of
board config file and existence/nonexistence of previous toolchain info.

Change-Id: I20e1794c12e7e60fdbbcd5a123ddc5c6bdf86b16
Reviewed-on: https://gerrit.chromium.org/gerrit/39132
Reviewed-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>
Commit-Ready: Ryan Cui <5ce1592d53754a87a44fc2eb7b63d5f77cf50fdb@chromium.org>
Tested-by: Ryan Cui <5ce1592d53754a87a44fc2eb7b63d5f77cf50fdb@chromium.org>

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