kbuild, deb-pkg: refactor code to reduce duplication

Factor out code to build package into separate function and
only write "source" section for the debian/control file once.

Signed-off-by: Frans Pop <elendil@planet.nl>
Acked-by: maximilian attems <max@stro.at>
Cc: Andres Salomon <dilinger@debian.org>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>