ToysRUs has updated their "Movie HQ" website with a section for Transformers: The Last Knight, and it answers a fair number of questions! The promotional image showing Optimus Prime and Megatron fighting on Cybertron is part of a text-heavy graphic on the site titled "Mission to Cybertron," which has a serious chunk of backstory on it that may or may not be SPOILERS for part of the TF5 plot. Probably not, but better safe than sorry, yes? Keep reading to see!
Given that no other outlet has been given this special title and logo to go with its products, it seems somewhat likely that the rather extensive line of TRU exclusives that have been leaked over the last few months will be branded as the Mission To Cybertron subline. Presumably, we'll find out closer to June 23rd when TLK hits theaters.