I've lost track of what is available on the piCore repos. The firmware packages are quite old. But the firmware is loading fine, according to the dmesg.
921600 noflow are not correct parameters for rpi0w. /usr/local/bin/hciattach /dev/ttyAMA0 bcm43xx 3000000 flow - b8:27:eb:xx:xx:xx
Use the script that I posted earlier....that is how they do it on Raspbian too. Also, make sure that you are not starting a serial console on ttyAMA0, as that would conflict with bluetooth.
AFAIK, the only extension related to bluetooth that doesn't run on a armv6 board is the sbc.tcz extension, that is related to bluetooth audio. I build all of the bluetooth packages, rebuilt upto bluez5.50