How to enable serial access to Raspberry Pi bluetooth chip ?

Hi,

There is a cool python-based bluetooth stack called "Bumble" created by 
Google folks,
which I have been using with my Intel NUCs (with FreeBSD) and with 
Raspberry Pi (with Linux)
to talk with my custom bluetooth stuff.

The stack talks directly to chip, using usb or serial.

I would like to use FreeBSD on Raspberry Pi for this too, but it seems 
that the bluetooth chip
is disabled there and I assume that in /boot/msdos/config.txt the line

dtoverlay=disable-bt

is related to this. I tried just removing that, but it resulted in a 
system which didn't boot.
I'm under impression that something is shared between bluetooth chip and 
serial console,
but i could be completely wrong with that. But I won't need the serial 
console if it
must be disabled to gain access to bluetooth.

I'm currently out of ideas with this so if anyone has some tips on how 
to proceed
I would very much appreciate those.

..
      Ari Suutari