kbuild: fix the binrpm-pkg target to work with KBUILD_OUTPUT set

The binrpm-pkg target (binary RPM only) fails when called with
KBUILD_OUTPUT set. This patch makes it work.

For the rpm-pkg target (source + binary RPM), building with
KBUILD_OUTPUT set is not possible and also not needed as the
actual build is done in a temporary directory anyway, so check
that KBUILD_OUTPUT is not set in that case to avoid later errors.

Signed-off-by: Frans Pop <elendil@planet.nl>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>