Use the last two parts of the archive path to determine version string.

On builders, cros_generate_test_payloads may run sequentially for two
different boards with the same version. Archive paths are given with
.../board_config/version. Use the last two parts and not just the version
when caching.

BUG=chromium:328615
TEST=Unittests

Change-Id: I57be88b09f0d26211f0e8799c81e07e40fb2cf2b
Reviewed-on: https://chromium-review.googlesource.com/180683
Tested-by: Chris Sosa <sosa@chromium.org>
Reviewed-by: Yu-Ju Hong <yjhong@chromium.org>
Commit-Queue: Chris Sosa <sosa@chromium.org>
2 files changed