Currently the settings for i.MX8 are fixed to only allow to use
OSMesa for offscreen rendering. Actually etnaviv is usable on
i.MX8MM an it should therefore be possible to let Gallium be
enabled instead of unconditionally disabling it.
Therefore we introduce the USE_OSMESA_ONLY variable, that can be
set to "no" for i.MX8 if etnaviv should be used just like for i.MX6.
