libkmod, depmod, modprobe: Make directory for kernel modules configurable

libkmod, depmod, modprobe: Make directory for kernel modules configurable

Now that modprobe.d is searched under ${prefix}/lib, allow a complete
transition to files only under ${prefix} by adding a ${module_directory}
configuration. This specifies the directory where to search for kernel
modules and should match the location where the kernel/distro installs
them.

With this distributions that do not want to ship files in /lib can also
move kernel modules to /usr while others can keep them in /lib.

Signed-off-by: Michal Suchanek <msuchanek@suse.de>
Link: https://lore.kernel.org/r/a3765f4e8ae3ce29c0847a0132d4a8d51ad040a9.1699618135.git.msuchanek@suse.de
Signed-off-by: Lucas De Marchi <lucas.de.marchi@gmail.com>