South Slavic name from 820th

I'v been regulary watching this group for a while because is very
interesting, although I am amateur in linguistics.
Linguistics is probably the only way to reach at least something of
ancient unwritten history.
Piotr is my "idol" here, so I would ask him kindly (or anybody else)
one little (maybe stupid) question.

Einhard in his anales mentions one slavic princ in 820th Croatia
under name Borna.
I always asked myself what is the meaning of that name. Is it posible
that this have something with later slavic names Brana or Branko.
Is it there metathesis of liquids in work.
And could the names Porin and Porga (greek source - Porfirogenitus)
have something in common with Borna, because that could be the same
person?
Thank's in advance.

Pozdrav svima!