kbuild, deb-pkg: fix 'file not found' error when building .deb package for arm

Not all architectures prepend the $(boot) path in $(KBUILD_IMAGE).
Allow for that fact in the builddeb script. Example is arm.

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>