kbuild, deb-pkg: fix generated package name

The binary package that make deb-pkg creates is a linux-image.
To be fixed may also be the addition of $DEB_ARCH.

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