package/mesa3d: re-arrange menu items

Make mesa3d a menuconfig, ie. a sub-menu on the boolean option.

Move drivers and additional API selections out of their own submenus,
add a comment as separator.

This will make it easier to further re-arrange the config items in
the following patches, to fix-enhance the packaging to handle the
full-openGL provider stuff.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Paul Cercueil <paul@crapouillou.net>
Cc: Bernd Kuhls <berndkuhls@hotmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>