bthelper: Add the ability to set the Pi BDADDR

If Bluetooth is being enabled via the kernel, hciattach isn't run and
we lose the ability to set the BDADDR. Extend bthelper to use the
bdaddr utility on Pis that haven't had the BT section of the OTP
programmed.

Also ensure that btuart doesn't do anything unless necessary.

Signed-off-by: Phil Elwell <phil@raspberrypi.com>