They are floating around. Some of them are in flight to land in mainline, some are WIP in heavy flux. In terms of functionality, mainline would not be able to do more than legacy so far. And since your offered legacy firmware works so far, there is currently no reason to do the work now to use the current mainline version. It is only interesting for someone who wants to add e.g. VOP2 or NVME driver support and ports the drivers from Linux kernel. For all pure users it is easier to wait until all outstanding components have landed and then just use mainline. Anyway, my SPEC file for the uboot-tools package is now ready to build M1 firmware as soon as needed. However, this will certainly take until 2023.07 at the earliest, because there are still some components under discussion and 2023.04 should be available next week.