gnu: linux-libre: Add i686 and x86_64 full-blown configs.

gnu: linux-libre: Add i686 and x86_64 full-blown configs.

Thanks to Jason Self <jself@gnu.org> for the kernel configs.

* gnu/packages/linux.scm (kernel-config): New procedure.
  (linux-libre)[build-phase]: Copy it to .config.  Reduce the list of
  things appended to .config.
  [native-inputs]: Add "kconfig" input.
* gnu/packages/linux-libre-i686.conf,
  gnu/packages/linux-libre-x86_64.conf: New files, from
  <http://jxself.org/x86-32.txt> and <http://jxself.org/x86-64.txt>.
* Makefile.am (KCONFIGS): New variable.
  (nobase_dist_guilemodule_DATA): Add it.