Give Bluetooth its factory address from the DPP partition

The WCN3990 has no burned-in Bluetooth address, so the kernel registers
the controller unconfigured and bluetoothd never sees it. The factory
address is on the machine all along: QCOM/BT.PROVISION on the DPP
partition, a three-byte header followed by the six-byte address.
c630-bt-addr reads it and hands it to btmgmt before bluetooth.service
starts. Verified on hardware: controller configures, powers, scans, and
streams A2DP.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011XgGF5wfxLDAybVnNz6eNQ

build/stage2.sh

@@ -284,6 +284,7 @@ chroot "$ROOTFS" passwd -l root
 # without tqftpserv, since the modem fetches the WLAN firmware over TFTP.
 chroot "$ROOTFS" systemctl enable \
     c630-growfs.service \
+    c630-bt-addr.service \
     sshd.service \
     NetworkManager.service \
     systemd-resolved.service \