buildroot

weston: add weston-launch support

The weston-launch supplemental group is for users to be able to
use/launch weston.
For a full weston(-launch) experience users should be at least members
of weston-launch, video (for framebuffer permissions) and optionally
audio.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>