Nice. Can you extend it so the serial port number, "#define DEBUG_PORT PORT_SERIAL1", in $seabios_dir/src/output.c is shared with CONFIG_CONSOLE_SERIAL_COM1 in coreboot's .config, and so on for COM2..4 ?
Dito for "#define CONFIG_DEBUG_LEVEL 1" in $seabios_dir/src/config.h and "CONFIG_DEFAULT_CONSOLE_LOGLEVEL=8" and/or CONFIG_MAXIMUM_CONSOLE_LOGLEVEL in .config.

Using releases ( http://seabios.org/Releases ) instead of code from seabios' git repo sounds like a better idea to me.

Idwer