I haven't looked into the character beyond what I saw in the trailers. I just found it strange that they would lerave out Luke from the poster if he is in the film.
That tends to happen if there's something about the character's appearance or involvement that needs to remain concealed for story purposes. And there we have a masked lightsaber weilding character, so I thought it'd fit.
Though not that I think this is neccesarily the case, but even if that character has a different face, that doesn't rule out that it could still be Luke.
He could be dressing up like this hooded man at some point in the film, so the character on the poster could represent both of them.
Or perhaps he went undercover and wears a mask or something, etc.

I think most people are expecting Luke to be in it, since Han and Leia are. So omitting him from the poster wouldn't really do much in terms of lowering people's expectations in that regard, I suspect. Perhaps there is something about his appearance that they want to conceal. But they could have just shown something like the backside of a person holding a green lightsaber in that case. A bit of an odd choice to not include such a huge icon on the promo poster.

Mixed race people like Obama are usually a paler colour though, can't really say the same for Boyega. Besides there's also the fact that he was a Stormtrooper. Don't think Luke would have let his son join the Empire

Well I don't think the size of Luke's role in the story would warrant removing such a huge icon from the poster. I forgot that we saw Luke's hand in the trailer, and apparently we also hear his voice at the end of the second one. Which I guess most people would see as confitrmation that he's in the film. But the poster seems to suggest that they still want to surprise people with the fact that he'll appear in it visually, and perhaps also by how he now looks. Though the latter could have easily been obscured in the poster, while just showing his lightsaber.

If this isn't the final 'main poster' for the film, then it's not so weird. I think there tends to be at least two. One for the theatres, and one for the DVD/Blu-Ray. But some times they're the same. Though I wonder if they'd really promote the theatrical version differently, by not spoiling Luke's involvement, while doing so on the Blu-Ray release.
I'm hoping we'll see Luke in the next trailer, which comes out tomorrow I think. And that they'll put him into the final poster.

That he might die like Obi Wan sounds like a plausible theory.