doc/develop/codingstyle.rst: Add a section on conditional compilation

doc/develop/codingstyle.rst: Add a section on conditional compilation

In order to make a start on explaining how and when to use certain
macros, we need to document their usage somewhere. As a first step, take
section 21 of the v6.13 Linux Kernel coding-style document on
conditional compilation, verbatim, and add it to our documentation.
Further rewording to be clearer about U-Boot will be done next.

Signed-off-by: Tom Rini <trini@konsulko.com>