UBUNTU: [Packaging] startnewrelease: Add support for backport versioning

UBUNTU: [Packaging] startnewrelease: Add support for backport versioning

Ignore: yes

Use the parent kernel version as base for the new release version if
the BACKPORT_SUFFIX variable is set in the update.conf file.

Signed-off-by: Marcelo Henrique Cerri <marcelo.cerri@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>