'make deb-pkg' builds build-arch in parallel, but binary-arch serially.

Given that all binary packages are independent of one another, they can
be built in parallel.

I am uncertain whether debian/files is robust against a race condition.
Just in case, make dh_gencontrol (dpkg-gencontrol) output to separate
debian/*.files, which are then concatenated into debian/files.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <n.schier@avm.de>