kheaders: include only headers into kheaders_data.tar.xz

Currently, kheaders_data.tar.xz contains some build scripts as well as
headers. None of them is needed in the header archive.

For ARCH=x86, this commit excludes the following from the archive:

  arch/x86/include/asm/Kbuild
  arch/x86/include/uapi/asm/Kbuild
  include/asm-generic/Kbuild
  include/config/auto.conf
  include/config/kernel.release
  include/config/tristate.conf
  include/uapi/asm-generic/Kbuild
  include/uapi/Kbuild
  kernel/gen_kheaders.sh

This change is actually motivated for the planned header compile-testing
because it will generate more build artifacts, which should not be
included in the archive.

Change-Id: I9df4bd8ab4b771051ad063035bd7b48b280b018f
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Git-commit: 199ff7d74003b5aad1e6328bf6128cd8ceea735  
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Gaurav Kohli <gkohli@codeaurora.org>