[coreboot] RFC: Replacing IS_ENABLED(CONFIG_XXX) with the shorter CONFIG(XXX)

[Julius Werner]

Hi folks,

I'm proposing a small policy change for the way we refer to boolean Kconfig
variables in code. Right now, the directive is to always use the
IS_ENABLED() macro. I would like to replace this with a shorter macro
CONFIG() that also removes the need to type the CONFIG_ prefix again. The
idea is mostly to save some horizontal space and the occasional extra line
break for this boilerplate and make it a little easier to type.

It's a very simple change (patch train starts at
https://review.coreboot.org/c/coreboot/+/31773), but since it affects
pretty much all code in coreboot and payloads I wanted to send out a quick
FYI. Please let me know if anyone has any concerns with this.