gnu: linux-libre: Update to 4.18.1.

gnu: linux-libre: Update to 4.18.1.

* gnu/packages/linux.scm (%linux-libre-version): Update to 4.18.1.
(%linux-libre-hash): Update hash.
(%linux-libre-patches): New variable.
(linux-libre): Add %linux-libre-patches.
* gnu/packages/aux-files/linux-libre/4.17-arm.conf,
gnu/packages/aux-files/linux-libre/4.17-arm64.conf,
gnu/packages/aux-files/linux-libre/4.17-i686.conf,
gnu/packages/aux-files/linux-libre/4.17-x86_64.conf: Delete files.
* gnu/packages/aux-files/linux-libre/4.18-arm.conf,
gnu/packages/aux-files/linux-libre/4.18-arm64.conf,
gnu/packages/aux-files/linux-libre/4.18-i686.conf,
gnu/packages/aux-files/linux-libre/4.18-x86_64.conf: New files.
* Makefile.am (AUX_FILES): Update accordingly.