The wiki is being retired!
Documentation is now handled by the same processes we use for code: Add something to the Documentation/ directory in the coreboot repo, and it will be rendered to https://doc.coreboot.org/. Contributions welcome!
LinuxBIOS in itself is "only" minimal code for initializing a mainboard with peripherals just enough for a Linux kernel to take over and to the rest. LinuxBIOS does not contain a kernel per se.
After the initialization, LinuxBIOS jumps to a payload and while there has been discussion about stacking payloads that's currently not in practice.
The payload was originally intended to be a Linux kernel stored in flash. Flash ROM grow rate was anticipated optimistically however, today there are not many mainboards that actually have enough flash ROM room for a kernel. 512KB can be seen here-and-there and a few boards come with 1MB. Recent kernels really want that MB, and then you'll only have room for 3-400 KB of initial ramdisk, which could be too small too, depending on the application.
So, other payloads are used; the two major ones are FILO and Etherboot. FILO loads a kernel from a filesystem on an IDE device and Etherboot loads a kernel from the network or from a filesystem on an IDE device.
If you're using FILO there is no Linux kernel until FILO loads it, and the kernel loaded by FILO (or Etherboot) can absolutely be the one you want to run in your system. Just set it up with the correct root and init commandline so that it can start init.
Etherboot --- it includes FILO, and its FILO support SATA and USB booting.
FILO - Simple bootloader with filesystem support