I've documented the bootloader protocol a bit more in the GitHub repo: https://github.com/tinyfpga/TinyFPGA-B-Series/blob/master/bootloader/README.md

If you want to integrate the TinyFPGA B-series into a development platform, you can ideally use the tinyfpgab.py programmer script instead of writing your own.  However, there are some cases where it's just not practical or user-friendly to do so.  In those cases this README should help along with the reference implementation that tinyfpgab.py provides.