package/mesa3d: add comment for when OpenGL EGL is not available

OpenGL EGL needs udev support, so add a comment stating so.

Also, reword the OpenGL EGL prompt to be in the same format as the
OpenGL ES prompt (and like the full OpenGL prompt that will shortly
be added, too.)

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>