kbuild: rpm-pkg: invoke the kernel build from rpmbuild for binrpm-pkg

To reduce the preprocess of the spec file, invoke the kernel build
from rpmbuild.

Run init/build-version to increment the release number not only for
binrpm-pkg but also for srcrpm-pkg and rpm-pkg.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>