The latest SeaBIOS git now has a feature which permits one to configure the default boot order via a CBFS file. This feature was added for QEMU, but it was done in such a way that Coreboot users can benefit as well.
To use the feature, see the documentation at:
http://www.coreboot.org/SeaBIOS#Configuring_boot_order
For other information on obtaining and using SeaBIOS, see:
http://www.coreboot.org/SeaBIOS
-Kevin