Since the movie deals with the death of T'Challa the song will be representative of the loss of both the actor and the character to which he portrayed.
As noted by Tems the co-writer of the song:
“After speaking with Ryan and hearing his direction for the film and the song, I wanted to write something that portrays a warm embrace from all the people who I’ve lost in my life. I tried to imagine what it would feel like if I could sing to them now and express how much I miss them,” Tems said in a statement. “Rihanna has been an inspiration to me, so hearing her convey this song is a great honor.”
So the idea of the song is about loss, and conveying the emotion felt by that loss. Now obviously I don't know the lyrics, but my guess is there won't be any direct naming of Boseman in the song, but rather a reference to "king". This works with both the actor and the character.
So yes the song is a tribute to Boseman, but its also a tribute to the character he played and the loss that is felt by the passing of both.