kbuild: srcrpm-pkg: create source package without cleaning

If you run 'make (src)rpm-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, the .config was contained in the source tarball.

With this commit, the source rpm consists of separate linux.tar.gz
and .config.

Remove stale comments. Now, 'make (src)rpm-pkg' works with O= option.

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