I know this is a dead topic but its incomplete as the instructions are missing a step and I have been pulling my hair out for months (very part time) trying to get a firmware to load.

For anyone else looking how to do this the answer seems to be here but not specifying '-9' as the compression level for the gzip command:
https://groups.google.com/g/beagleboard/c/eMEPdEuAQjE?pli=1

I have been using the steps posted earlier with the gzip command like this to repack the firmware up:

find . | cpio -o -H newc > ../Angstrom-antminer_m-eglibc-ipk-v2013.06-beaglebone.rootfs.cpio
cd ..
gzip Angstrom-antminer_m-eglibc-ipk-v2013.06-beaglebone.rootfs.cpio
mkimage -n 'Angstrom-antminer_m-eglibc-ipk-v' -A arm -O linux -T ramdisk -d Angstrom-antminer_m-eglibc-ipk-v2013.06-beaglebone.rootfs.cpio.gz ../initramfs.bin.SD
cd ..
tar czvf Antminer-D3-myversion.tar.gz initramfs.bin.SD runme.sh am335x-boneblack-bitmainer.dtb

Hope this helps someone and may work for newer miners but I havent got one yet as still working on a PoC

Thanks
Adam