doc: kconfig: Add section on (lack of) conditional includes
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 <01743f1c1deb1fe75271d62edb65757d98ac0bcb@nordicsemi.no>

finikorg/zephyr,galak/zephyr,Vudentz/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,galak/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,ldts/zephyr,Vudentz/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,galak/zephyr,galak/zephyr,nashif/zephyr,nashif/zephyr,Vudentz/zephyr,finikorg/zephyr,Vudentz/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,Vudentz/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,ldts/zephyr,galak/zephyr