kbuild, deb-pkg: allow to specify a custom revision for .deb packages

Allow to specify a custom revision for the generated .deb by
exporting the environment variable KDEB_PKGVERSION.

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>