It sounds like you are trying to use github.com/sifive/freedom-u-sdk. I don’t know why you referred to unrelated software earlier.
Which version of freedom-u-sdk do you have? There are 3 distinctly different versions of it. The current version, the third version, is an Open-Embedded layer, using opensbi… The second version was a machine mode u-boot with linux kernel and buildroot and an optional debian rootfs. The first version, the one that ships with the board, was a minimal buildroot with bbl and linux kernel.
How you boot depends on which version you have. The boot select pins need to be in different positions for different versions. This is explained in the README.md file for the second and third versions of freedom-u-sdk. This is explained in the Getting Started Guide for the first version.
Since you mentioned opensbi, I think you have the third version. There is a diagram in the README.md file near the end that explains how to set the boot select pins to get it to boot.