cros_extract_deps: make --board optional

This allows the script to output dependencies for the packages on the
host.

BUG=chromium:422639
TEST=`cros_extract_deps virtual/target-sdk`

Change-Id: Ib5bfee2484e5e56d21df88ee14c8f834c2b3835a
Reviewed-on: https://chromium-review.googlesource.com/225663
Tested-by: Yu-Ju Hong <yjhong@chromium.org>
Reviewed-by: David James <davidjames@chromium.org>
Commit-Queue: Yu-Ju Hong <yjhong@chromium.org>
1 file changed