Where can I find the source code to the MCU’s firmware? hifive-premier-p550-tools/mcu-firmware at master · sifive/hifive-premier-p550-tools · GitHub just contains an ELF…
I want to check if it’s really as stupid as it looks in the user manual, that only a Web interface is accessible over ethernet, and not the CLI. 
It’s closed sourced AFAIK. You can sort of create a CLI yourself. The Web interface will use REST/json API to communicate with the MCU. E.g., to get power status, you can use curl -v http://<ip>/power_status (Other APIs can be easily discovered by using developer tools in the browser to capture network requests)
{
"status": 0,
"message": "success",
"data": {
"power_status": "0"
}
}
The code quality is pretty bad, though. I can sort of “crash” it (unresponsive to further requests) by doing a port scan through nmap. Luckily the MCU is a STM32F4 (STM32F407VET6) that has the tooling readily available. So I hope I can do a full open-source re-write some time later.
Currently MCU repo is private and we will provide fixes for firmware in form of binary.