coding_style: Add more guidelines on error handling, die() and assert()

coding_style: Add more guidelines on error handling, die() and assert()

This patch adds a new section to the coding style which codifies
existing practices about how to handle errors and how to use the die()
and assert() macros. Also clean up some references to Linux-specific
facilities that do not exist in coreboot in the adjacent function return
type guidelines, and add a small blurb of documentation to the
definition of the assert() macro itself.

Signed-off-by: Julius Werner <jwerner@chromium.org>
Change-Id: Ice37ed9f995a56d69476e95a352209041b337284
Reviewed-on: https://review.coreboot.org/c/coreboot/+/70775
Reviewed-by: Elyes Haouas <ehaouas@noos.fr>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Martin L Roth <gaumless@gmail.com>