gnu: Allow services to install kernel-loadable modules.

gnu: Allow services to install kernel-loadable modules.

* gnu/system.scm (operating-system-directory-base-entries): Remove code
to handle generation of "kernel" for linux-libre kernels.
(operating-system-default-essential-services): Instantiate
linux-builder-service-type.
(package-for-kernel): Move ...
* gnu/services.scm: ... to here.
(linux-builder-service-type): New variable.
(linux-builder-configuration): New type.
(linux-loadable-module-service-type): New variable.
* gnu/tests/linux-modules.scm (run-loadable-kernel-modules-test): Move
code to ...
(run-loadable-kernel-modules-test-base): ... new procedure here.
(run-loadable-kernel-modules-service-test): New procedure.
(%test-loadable-kernel-modules-service-0): New variable.
(%test-loadable-kernel-modules-service-1): New variable.
(%test-loadable-kernel-modules-service-2): New variable.
* doc/guix.texi: Document linux-loadable-module-service-type.

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