cros_setup_toolchains: add a helper for listing target packages

Other tools (notably update_chroot) want the list of packages for a
specific target, so add an option for it.

BUG=chromium:751852, b:162254118
TEST=precqs pass

Change-Id: I550aaea1c3f6fc5ce384354ed5a6c6d4b8c8c544
Reviewed-on: https://chromium-review.googlesource.com/664840
Commit-Ready: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Manoj Gupta <manojgupta@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/2478984
Tested-by: Patryk Duda <patrykd@google.com>
Commit-Queue: Patrick Georgi <pgeorgi@chromium.org>
Reviewed-by: Jett Rink <jettrink@chromium.org>
1 file changed