[coreboot] what if some mainboard use that function, others dont

[Bao, Zheng]

I need to add some functions in early_setup.c of southbridge, sb700, for
example. But some other mainboards don't use those functions. Given the
Werror in Makefile, I can't make these 2 cases pass the compiling both.
What is the resolution? Do I have to add a configuration in mainboard
Kconfig for such a trivial function?

Zheng