doc/develop/codingstyle.rst: Expand to include CONFIG_IS_ENABLED and PHASE_

Expand the conditional compilation section to explain when to use
CONFIG_IS_ENABLED rather than IS_ENABLED and provide an example. Next,
note what the PHASE_ macro is supposed to be used for as well.

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