UBUNTU: future-proof ddeb handling against buildd changes

At some point, Launchpad will gain the ability to process ddebs.  When
that happens, we'll add a flag to the /CurrentlyBuilding file on the
buildds.  It'd be nice to ensure in advance that the kernel packaging
can handle this, as pkg_create_dbgsym already does.

Signed-off-by: Colin Watson <cjwatson@ubuntu.com>
Signed-off-by: Andy Whitcroft <apw@canonical.com>