I wasn’t able to easily find information on how to add USB bootloader support. I have compiled my findings and hopefully you will find this very useful.
Can you please not use shorten URL? This is not twitter so there are NO length limits. Can't see where I am going *without* clicking on link first - major security concerns there. Also they can expire.
For compiling STM32 bootloader based on rogrogerclark's firmware found here: https://wiki.stm32duino.com/index.php?title=Bootloader, I have written down the detail steps to compile USB bootloader for STM32F103C8T6 (BluePill board) here: https://github.com/TamojitSaha/Compiling_Arduino_STM32_bootloaders
If found helpful, or if there are any part that needs some changes please contact me, I will try my best to resolve it.