bsd.rd fails to boot up on libreboot x200: how to find out why?

I have a Thinkpad X200 laptop with libreboot 20150518 (latest stable release). Libreboot is basically coreboot + grub2 without bios services and with no text console. GRUB is able to load openbsd kernels from all filesystems it knows including FFS. If I load 5.9/amd64/bsd or bsd.mp

Running OpenBSD 5.9 from libreboot without proprietary blobs is possible. Detailed instructions inside.

Libreboot is a free software BIOS replacement, see https://libreboot.org for details. It is a distribution of Coreboot without proprietary blobs, including CPU microcode. All tests were performed with Thinkpad X200, but it should work for most or all libreboot and autoboot machines. Since 5.9,