Hello Duncan,
There was a SeaBIOS upgrade from 1.10.x to 1.11.x some time ago. As You maybe already figured out, SeaBIOS needs to be configured to use serial console from now on. Try adding the following file to CBFS image by doing:
cbfstool coreboot.rom add-int -i <port> -n etc/sercon-port
The serial port should be 0x3f8.
I have some ALIX boards at hand, but I did not build a coreboot image for quite some time. They may be other issues before SeaBIOS gets loaded.
Let me know if that helped. Also try to recompile with loglevel SPEW so that I could tell anything more and if everything is allright.
Regards, MichaĆ