Pretty simple if potentially impossible, >80% of the landmass of Africa must be under a single nation, this nation must at the least have the same foreign policy, that's all. Pretty much the thinnest definition of nation.
You've got till 2020 to manage, it can even be under another nation (as the entire continent would still have the same foreign policy).
How (un)stable would this entity be? Where would it likely be unable to hold (Egypt? North Africa?)
Edit: Some ideas - some loud-mouthed idiot in Europe decides they want Africa all to be united or something, convinces some important people, possibly from racist platform?