gnu: Use make-linux-libre-headers.

gnu: Use make-linux-libre-headers.

* gnu/packages/linux.scm (make-linux-libre-headers): New variable.
(linux-libre): Rename to...
(linux-libre-5.1): ...this.
(linux-libre-headers): Rename to...
(linux-libre-headers-4.14.67): ...this.
(linux-libre-5.1, linux-libre-headers-4.14.67): Use make-linux-libre-headers.
(linux-libre-5.1, linux-libre-headers-5.1, linux-libre-headers-4.19,
%linux-libre-4.15-version, %linux-libre-4.15-hash, linux-libre-4.15,
linux-libre-headers-4.15, linux-libre-headers-4.14): New variables.

Signed-off-by: Danny Milosavljevic <dannym@scratchpost.org>