Hi Arthur,
I'm very much in favor of using #pragma once instead of the include guards in the coreboot tree, since that removes the possibility of some sorts of bugs and also removes 2 lines of boilerplate per header file. Not sure if romcc supported #pragma once and if that was one of the reasons to use the include guards instead, but since romcc was dropped from the main branch a long time ago, that's not a problem.
Regards, Felix