I'm not sure any of those are reboots, the originals are just continuations with timeline variance. The prometheus and covenant movies are prequels. Alien vs Predator is some hollywood cash grab and isn't a reboot in the true sense. Nostromo maybe but I wouldn't say its a reboot either.
When I think of reboot, and I might be wrong here, I think of the original story/protagonist getting rewritten with a change in the storyline to effect a new way of telling the same story. Batman is a good example of this. With the Alien series I feel like they've always taken it in different directions and even with Ripley two timelines never intersect its more of a continuation.