UBUNTU: lucid: parameterise base version search string
For OEM branches, we may have different version numbering in the release
commit messages. In this case, the current method of finding baseCommit
does not work, so printchanges can't find the correct set of commits to
generate the changelog.
This change parameterises the search string use to find baseCommit,
allowing it to be overridden by a branch-specific variable. If no
variable is provided, we default to the current string.
Signed-off-by: Jeremy Kerr <email@example.com>