[S] Change in coreboot[master]: contributing/coding_style.md: Add and clarify includes rules

[Julius Werner (Code Review)]

Attention is currently required from: Nico Huber, Martin L Roth, Paul Menzel, Arthur Heymans, Elyes Haouas.

Julius Werner has posted comments on this change. ( https://review.coreboot.org/c/coreboot/+/71612 )

Change subject: contributing/coding_style.md: Add and clarify includes rules
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Patch Set 1:

(4 comments)

File Documentation/contributing/coding_style.md:

https://review.coreboot.org/c/coreboot/+/71612/comment/dd292364_fe2ee5d3 
PS1, Line 921: Local "file.h" includes should always come separately after all
             : <file.h> includes
Order of local vs. system includes is already mentioned here. If you want to start making <vendorcode> a separate group (do we want that? Why?) you could add it here.

https://review.coreboot.org/c/coreboot/+/71612/comment/4409cf24_7b52135a 
PS1, Line 925: The includes should be ordered alphabetically.
Last time there was a big fight about this I think we decided that we don't want to mandate it.

https://review.coreboot.org/c/coreboot/+/71612/comment/30bdb0c0_178503ff 
PS1, Line 927: Don't use UNIX directory '.' or '..' shortcuts and put conditional includes
This doesn't really match existing practice in coreboot (e.g. mainboard directories often contain local headers that are included via double-quote paths, as explicitly allowed in the second sentence of this paragraph, and in cases where those headers are one directory up you have to use '..').

https://review.coreboot.org/c/coreboot/+/71612/comment/d7a64454_f8b64d93 
PS1, Line 933: #ifdef CONDITION
Honestly, I think conditionally including files is a pretty bad practice in general and we should just avoid it completely wherever possible. So I'm not sure it's a good idea to explicitly list that case here.