gnu: linux-libre: Update to 4.20, while retaining 4.19 LTS.

gnu: linux-libre: Update to 4.20, while retaining 4.19 LTS.

* gnu/packages/linux.scm (%linux-libre-version): Update to 4.20.
(%linux-libre-hash): Update hash.
(%linux-libre-4.20-patches): New variable.
(linux-libre): Use %linux-libre-4.20-patches.
(%linux-libre-4.19-version, %linux-libre-4.19-hash)
(%linux-libre-4.19-patches, linux-libre-4.19): New variables.
(linux-libre-arm-generic, linux-libre-arm-omap2plus): Use
%linux-libre-4.20-patches.
(linux-libre-arm-generic-4.19, linux-libre-arm-omap2plus-4.19): New variables.
* gnu/packages/aux-files/linux-libre/4.20-arm.conf,
gnu/packages/aux-files/linux-libre/4.20-arm64.conf,
gnu/packages/aux-files/linux-libre/4.20-i686.conf,
gnu/packages/aux-files/linux-libre/4.20-x86_64.conf: New files.
* Makefile.am (AUX_FILES): Add them.