mesa: merge the .bb content into .inc

This allows automated version updates to the recipe,
as mesa-gl.bb no longer has to include the main mesa.bb.

Bump mesa-gl to 20.0.7 at the same time.

Signed-off-by: Alexander Kanavin <alex.kanavin@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>