Hi Eli,
I've been using Coreboot on my board for several weeks, it is serving as an HTPC running 24/7 and it's working perfectly stable which suggests the firmware should be free of bugs. It is likely that you are facing some configuration or hardware issue here.
I didn't see any issues with the serial output while working with the board once the SuperIO chip started to work. Make sure the cable you use is intact and try to attach it to another machine. At the time I was working with my board, I used a Dell Port Replicator with a native COM port so I could use a standard null-modem cable and it was working flawlessly. In case you use a USB serial adapter, try replacing it or attach the serial cable directly to a COM port if you have one available.
Also, please enable debug_level=Spew as it seems the console log you attached comes from a less verbose setting and therefore it's not as useful as it should be.
Note that the VBIOS image is executed by SeaBIOS which means you won't see anything on your display until the payload is executed.
If all else fails, you can still attach a POST debug module to the LPC header of the board [1] which can help a lot to find out where the boot process hangs.