Wall of Fire does double damage against undead.
An evoker's elemental wall "deals acid, cold, electricity, or fire damage, determined when you create it. The elemental wall otherwise functions like wall of fire."
Does the elemental wall do double damage to undead, regardless of energy type?
We can perhaps attempt to ascertain precedent from the admixture school:
"Versatile Evocation (Su): When you cast an evocation spell that does acid, cold, electricity, or fire damage, you may change the damage dealt to one of the other four energy types. This changes the descriptor of the spell to match the new energy type. Any non-damaging effects remain unchanged unless the new energy type invalidates them (an ice storm that deals fire damage might still provide a penalty on Perception checks due to smoke, but it would not create difficult terrain). Such effects are subject to GM discretion. You can use this ability a number of times per day equal to 3 + your Intelligence modifier."
Either we accept that this text is a general application (a), or we argue that because elemental wall does not have this text it ignores this limitation (b). I will discuss both.
(b) is simpler. The ability states it functions as wall of fire except it does a different damage type. There is no other clarification. Therefore, it still does double damage to undead.
(a) is a bit more complicated. Assuming we take versatile evocation as a precedent, is the extra damage to undead a property of the fire damage? This is ultimately DM purview, but I would strongly argue the two effects are not instrinsically related: after all, fireball, burning hands etc do not do more damage to undead and it doesn't spell out a specific property of fire causing the extra damage. The admixture text does mention that effects should remain unchanged unless specifically invalidated, which I don't see happening here. Therefore, changing the elemental type should still cause double damage to undead.
Conclusion:
The elemental wall should do double damage to undead, regardless of energy type and regardless of the logic applied.
By RAW, yes, as no part of Wall of Fire calls out that double damage as being in any way related to its damage type, and a quick perusal of other fire spells doesn't indicate that any other fire-related spell deals extra damage to undead. I can find no rule in the Bestiary that says undead are particularly damaged by fire. That may be how it works in many other rpgs, especially crpgs, but I would rule that Elemental Wall does indeed deal double damage to undead regardless of which element you chose. I won't hazard a guess as to the RAI.
I had never noticed the double damage to undead bit. Why is that even there? It's been a long time since undead as a type had a thematic weakness to fire.
I hadn't noticed it either, but it goes all the way back to OD&D.