Just a heads up - brd_pwm_count is still mentioned in hwdef.dat.
I don’t recall exactly how I did it, but I seem to recall having to look up this file to determine that the GPIO pin I needed for my camera shutter was 55.
With the changes to copter 4.2.0-rc1, I don’t know yet if there’s anywhere I’ll need to refer specifically to GPIO pin 55. Maybe the new parameter options handle this automatically now.