Hello,
From reading the Makefile in payloads/external/SeaBIOS, both CONFIG_CONSOLE_SERIAL and CONFIG_DRIVERS_UART_8250IO need to be set. However, I can't seem to find which config file I need to modify to properly set these variables. Additionally, CONFIG_TTYS0_BASE needs to be set as well. I've also attempted to manually build SeaBIOS with serial output enabled, but that didn't seem to have any impact on the result.
From what I know you will have to slightly modify the coreboot source to be able to personalize your seabios configuration. https://pelican.craoc.fr/coreboot.html#seabios_1 -- Regards Maxime de Roucy