system: Add kernel-loadable-modules to operating-system.

* gnu/system.scm (<operating-system>): Add kernel-loadable-modules.
(operating-system-directory-base-entries): Use it.
* doc/guix.texi (operating-system Reference): Document
KERNEL-LOADABLE-MODULES.
* gnu/build/linux-modules.scm (depmod): New procedure.
(make-linux-module-directory): New procedure.  Export it.
* guix/profiles.scm (linux-module-database): New procedure.  Export it.
* gnu/tests/linux-modules.scm: New file.
* gnu/local.mk (GNU_SYSTEM_MODULES): Add it.
* gnu/packages/linux.scm (make-linux-libre*)[arguments]<#:phases>[install]:
Disable depmod.  Remove "build" and "source" symlinks.
[native-inputs]: Remove kmod.