If the Jedi are a myth, then Luke is probably in hiding.
And it's up to Han and the crew to find Luke. We'll probably find him sitting in front of that fire with R2-D2 for company. Luke's absence is telling but I doubt he's on the side of evil.
This seems to gel best with the trailer and the way the ROTJ seemed to end. Luke seemed very world weary. But he swore he would "never turn to the dark side", and he also threw away his lightsaber.
Seems possible that between Lukes reluctance to risk falling to the dark side himself, and the awkward situation he would find himself in being both the last of the jedi AND Vaders son, he decided to sit a few decades out. Given how hard the empire tried to stamp out any memory of the jedi I imagine the entire aspect of the force and the jedi/sith would have faded from collective knowledge, especially with the upheaval that continued.
And now with the force "awakening" he is going to be drawn back into it again....