make: install/uninstall tools symlinks to kmod

make: install/uninstall tools symlinks to kmod

Currently we create symlinks like modprobe (pointing to kmod), during
the normal `make` build. Although those were never installed.

Add a few lines in the install-exec-hook, to ensure they're present at
`make install` time. Thus one can actually use those without additional
changes. As an added bonus, distributions can drop the similar hunk from
their packaging.

Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
[ Use a relative symlink ]
Signed-off-by: Lucas De Marchi <lucas.de.marchi@gmail.com>