buildbot: Add legacy_image.bin into firmware archive.
buildbot: Add legacy_image.bin into firmware archive.

The legacy_image.bin is also helpful for firmware testing and releasing.

BUG=chrome-os-partner:7952
TEST=trybot:
     http://chromesshgw/i/tryserver.chromiumos/builders/link-release/builds/10

Change-Id: I8c727738c4e3e22adb45b0a1a73f8a82ba8cfd01
Reviewed-on: https://gerrit.chromium.org/gerrit/22344
Tested-by: Hung-Te Lin <8a7fce4940baad93a69f5073b383c85deed9d9c4@chromium.org>
Reviewed-by: Vic Yang <5fd92abeee651458f98e2856f4f200ee971cee4d@chromium.org>
Commit-Ready: Vic Yang <5fd92abeee651458f98e2856f4f200ee971cee4d@chromium.org>

chadversary/chromiumos.chromite,coreos/chromite,zhang0137/chromite,chadversary/chromiumos.chromite,zhang0137/chromite,bpsinc-native/src_third_party_chromite,bpsinc-native/src_third_party_chromite,coreos/chromite,coreos/chromite,bpsinc-native/src_third_party_chromite,zhang0137/chromite