AMD's Olive Hill+ development board for Steppe Eagle should be bootable using Coreboot. Olive Hill is officially known as the AMD DB-FT3b and AMD DB-FT3. These boards come in a mini-ITX form factor and feature dusla DisplayPort, USB 3.0 and USB 2.0, an integrated platform security processor, DDR3 memory support, dual SATA, SD card, PCI Express x4, and mini Express.
The Coreboot enablement of Olive Hill came again through Sage Electronic Engineering, the firm contracted by AMD that does much of their Coreboot work. While the basic support is there, this nearly three thousand lines of new Coreboot code doesn't yet handle S3 and fan control. HD audio also isn't working.
More details on the Coreboot Olive Hill support via this Git commit.