what is the bootup flow of the default freedom-u-sdk on unleased board? any documents?
At https://www.sifive.com/documentation you can find a Unleashed Getting Started Guide that has some info on boot flow, along with info on how dip switch settings change it.
You can also find the zsbl and fsbl sources at https://github.com/sifive/freedom-u540-c000-bootloader and read them to learn the ugly details.