SZL no longer do self-extraction for runtime binary, it would boot from
SD/ethernet depending on the boot mode settings.
This allows a larger runtime binary, so we can optimize for speed in the
runtime firmware for better performance, and allow more features to be
added later.
* based on https://github.com/jethrogb/rust-core_io but could not get the packaging scripts to work
and the repos is unmaintained anyway, so just copied the result
* more features and more up-to-date
* compatible with the fatfs crate
