This implements rebooting to bootloader via os.reset(True). To do that we modify the bootloader so that it checks for a special file named .mscmode and if it's there it removes it and runs msc(). On Python side we declare .mscmode as a restricted file and create it before rebooting if needed.
This also improves update-bootloader.bin building because I didn't get a debugger :D It now automatically uses the latest bootloader, so you can grab update-bootloader.bin.