They probably

want to let your boss go. They just want to make sure to hear opinions from all sides(since you were promoted, they thought you may on your boss' side). They want to make sure their decision is justified.

Tell the truth, it's better for you that your boss is gone. The reason of your promotion was probably because that your boss wanted to prove that he/she was objective and fair, as he/she didn't dislike everybody. You were probably the one he/she dislikes the least.