^ Not in serialized fiction; the term reboot, when used in that context and medium, means to discard and start over.
Since you're way too hung up on the definition of the word "reboot," let's use another word.
Singer also just went on record stating that he is not ignoring any of the existing stories with DoFP, although both First Class and The Wolverine appear to have already ignored Origins: Wolverine, so we'll have to see what happens with DoFP for sure.
And again, I'm not saying they should ignore anything. I'm saying that time travel is a tool that can be used to make changes to the currently-existing X-Men universe. Perhaps, through the course of DOFP, certain things will end up differently than they were at the end of First Class
or The Last Stand
(for instance, maybe at the end of the movie, Professor X, Scott, and Jean are all alive and well in their new altered timeline).