The simplest and least hassle route would be Morocco - Mauritania - Mali - Burkina Faso - Ghana. There are some recent posts about this route in the last two months if you search the HUBB.
On a motorbike there is definately no need for a carnet on this route, although recent reports suggest that you might need a carnet for Ghana if you enter with a car. Aside from that, there arent any real hassles with borders or road conditions following this route.
cheers
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