micronucleus | Experimental features for attiny-based bootloader

micronucleus is a C library. micronucleus has no bugs, it has no vulnerabilities and it has low support. However micronucleus has a Non-SPDX License. You can download it from GitHub.

I based this on a USBaspLoader rewrite I'm working on, though it's pretty similar to the current micronucleus. A central goal is code clarity, to allow getting the fundamentals working really well. The large number of options in the main version overwhelm me. All received USB data's checksum is verified before it's operated on, including bytes in the protocol headers. Reset vector can be patched in bootloader (adds 46 bytes) or on host. In latter case major version is bumped to 2 and host handles this differently. Since CRC is now checked, only bugs in host program can cause wrong reset vector to be written. I'm including this option mainly as a proof-of-concept to evaluate. Flash erase is simplified to just erasing everything below the bootloader, from end to beginning. As far as I can tell, it's still robust to interruption.