gnu: linux: Add linux-libre arm kernel.

* Makefile.am (AUX_FILES): Add arm config file.
* gnu/packages/aux-files/linux-libre/4.13-arm.conf: New file.
* gnu/packages/linux.scm (%intel-compatible-systems): Rename to
  %linux-compatible-systems and add "armhf-linux".
  (linux-libre): Adapt consequently.

The config file for arm is taken from debian armmp kernel. It's a generic
firmware for ARMv7 multiplatform compatible SoCs.