kbuild: deb-pkg: create source package without cleaning

If you run 'make deb-pkg', all objects are lost due to 'make clean',
which makes the incremental builds impossible.

Instead of cleaning, pass the exclude list to tar's --exclude-from
option.

Previously, *.diff.gz contained some check-in files such as
.clang-format, .cocciconfig.

With this commit, *.diff.gz will only contain the .config and debian/.
The other source files will go into the .orig tarball.

linux.tar.gz is rebuilt only when the source files that would go into
the tarball are changed.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nicolas Schier <nicolas@fjasle.eu>