package/libcgroup: allow to build cgroup-tools

Add a new config option that allows to build cgroup-tools - a set of
command-line utils for managing cgroups.

[Thomas: slightly change the prompt of the new Config.in option.]

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Reviewed-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>