doc: kconfig: Add section on (lack of) conditional includes

Add a section on how 'if' works to the 'Kconfig - Tips and Best
Practices' page, especially when combined with 'source'.

There seems to have been some confusion here that lead to a bunch of
duplicated dependencies. It's probably simpler than most people assume.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>
1 file changed