buildbot: try to figure out a real version for workon uprevs
buildbot: try to figure out a real version for workon uprevs

This uses a script that can be provided per ebuild (named
chromeos-version.sh under the $FILESDIR directory). It is expected to return
the version number for the package (or no output in case of failure or
unknown version).

If the script doesn't exist, the same behavior as before is used.

BUG=chromium-os:27453
TEST=Manual inspection of the chromeos-kernel ebuild version string
compared with the contents of the first 3 lines in the Makefile in
the repo.

Change-Id: Ia83ffec6b759e03240ee84361994efc88ca35ce6
Reviewed-on: https://gerrit.chromium.org/gerrit/17439
Tested-by: Olof Johansson <13dc431ca3b0f062f888def8f8ec4662ec608f7d@chromium.org>
Reviewed-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>
Commit-Ready: Olof Johansson <13dc431ca3b0f062f888def8f8ec4662ec608f7d@chromium.org>
Reviewed-by: Chris Sosa <959d3d631fb6f17f1b07f751372041a1edb210d1@chromium.org>

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