Change in coreboot[master]: payloads/external/tianocore: Add build argument for 4G Decode

[Nico Huber (Code Review)]

Attention is currently required from: Sean Rhodes, Martin Roth, Matt DeVillier, Andy Pont, Paul Menzel, Stefan Reinauer, Angel Pons.
Nico Huber has posted comments on this change. ( https://review.coreboot.org/c/coreboot/+/56223 )

Change subject: payloads/external/tianocore: Add build argument for 4G Decode
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Patch Set 20:

(2 comments)

File payloads/external/tianocore/Kconfig:

https://review.coreboot.org/c/coreboot/+/56223/comment/cc4730b9_91c7aca8 
PS16, Line 86: 	bool "Avoid above 4G memory"
             : 	default n
             : 	help
             : 	  Select this option to not reserve memory above 4G. This will allow
             : 	  the payload to use all of the memory.
> > How does it work if Above4GDecode() is not called? What is […]

The current version says this:

config TIANOCORE_ABOVE_4G_DECODE
    	bool "Enable above 4G memory"

That's two-times incosistent: 1. it uses the term `decode` which is not
related to what the option does. 2. it says `decode` in one line and `memory`
in the next. So I would just change it to `TIANOCORE_ABOVE_4G_MEMORY`.
No confusing `Decode` or `EFI` or `Compatibility` or `Mode` needed, why not
just name things by what they do?

File payloads/external/tianocore/Kconfig:

https://review.coreboot.org/c/coreboot/+/56223/comment/f06d1215_ed29bbe2 
PS20, Line 92: reserve memory above 4G
I fear this was written after my misunderstanding of what is `reserved`.
Usually we reserve memory to make sure it's not used (for something else),
but it seems in this case memory is reserved for tianocore, so only the
reserved memory will be used. (I would call it allocation, actually.)