[S] Change in flashrom[main]: serprog: Add support for multiple SPI chip selects

[Anastasia Klimchuk (Code Review)]

Attention is currently required from: Riku Viitanen.

Anastasia Klimchuk has posted comments on this change. ( https://review.coreboot.org/c/flashrom/+/80498?usp=email )

Change subject: serprog: Add support for multiple SPI chip selects
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Patch Set 2:

(3 comments)

Patchset:

PS2: 
Riku, thank you for your patch! I have two small comments.

File serprog.c:

https://review.coreboot.org/c/flashrom/+/80498/comment/3b6802cf_d96d22fa :
PS2, Line 827: cs = extract_programmer_param_str(cfg, "cs");
Since you are adding new programmer param, you need to update manpage as well.

Specifically, manpage is generated from the file `doc/classic_cli_manpage.rst`, look for the section called `serprog programmer` and you can add a paragraph about `cs` param.
May be useful to mention where it is supported (from your code, there are error paths, seems like it's not supported everywhere).
Thank you!

https://review.coreboot.org/c/flashrom/+/80498/comment/bfbb8cc9_2cc8bc0d :
PS2, Line 833: msg_perr("Error: Invalid chip select requested! "
             : 				         "Only 0-255 are valid.\n");
Not critical comment, but: this can be one line (and same for another error message before).

Our guidelines for line length are here: https://www.flashrom.org/dev_guide/development_guide.html#coding-style