kbuild: do not create intermediate *.tar for source tarballs

Since commit 05e96e96 ("kbuild: use git-archive for source package
creation"), a source tarball is created in two steps; create *.tar file
then compress it. I split the compression as a separate rule because I
just thought 'git archive' supported only gzip.

For other compression algorithms, I could pipe the two commands:

  $ git archive HEAD | xz > linux.tar.xz

I read git-archive(1) carefully, and I realized GIT had provided a
more elegant way:

  $ git -c tar.tar.xz.command=xz archive -o linux.tar.xz HEAD

This commit uses 'tar.tar.*.command' configuration to specify the
compression backend so we can compress a source tarball on-the-fly.

GIT commit 767cf4579f0e ("archive: implement configurable tar filters")
is more than a decade old, so it should be available on almost all build
environments.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>