[coreboot] [RFC] #pragma once

[Arthur Heymans]

Hi

To make sure headers don't create conflicts, guards are added to all of
them.
But the guard needs to be correct: e.g.
https://review.coreboot.org/c/coreboot/+/64360/2
Most compilers implement '#pragma once ' as an alternative.
Should we use this instead across the tree, as it is less error prone and
less code?

Sidenote: clang warns about wrong header guards.
https://review.coreboot.org/c/coreboot/+/62173/23 hooks up clang to our CI
for some platforms ;-).

Kind regards
Arthur