payloads/external/SeaBIOS: Add option for saved SeaBIOS .config

Instead of adding various SeaBIOS options into the coreboot Kconfig,
just add a way to use saved SeaBIOS .config files.  These files
can contain full SeaBIOS .configs, but is really intended for individual
options.

The coreboot Kconfig options take precedence over the settings in the
saved .config.

Change-Id: Ia7f9c76555b8e290777207b3f637c94c4d67a782
Signed-off-by: Martin Roth <martinroth@google.com>
Reviewed-on: https://review.coreboot.org/12568
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Alexander Couzens <lynxis@fe80.eu>
3 files changed