doc: provide board porting guidelines
doc: provide board porting guidelines

Provide board porting guidelines to help achieving a coherent
board library to ease work for people developing application
not board specific and define clear objective to help board
porting reviews.

Signed-off-by: Erwan Gouriou <de4d967452b83bb0e37525652734e7d9163b6a4c@linaro.org>

nashif/zephyr,explora26/zephyr,Vudentz/zephyr,explora26/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,nashif/zephyr,nashif/zephyr,finikorg/zephyr,Vudentz/zephyr,zephyriot/zephyr,kraj/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,mbolivar/zephyr,mbolivar/zephyr,punitvara/zephyr,nashif/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,explora26/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,galak/zephyr,zephyriot/zephyr,galak/zephyr,Vudentz/zephyr,punitvara/zephyr,zephyriot/zephyr,punitvara/zephyr,nashif/zephyr,zephyriot/zephyr,galak/zephyr,kraj/zephyr,mbolivar/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,explora26/zephyr,explora26/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,ldts/zephyr,finikorg/zephyr,Vudentz/zephyr,mbolivar/zephyr,zephyriot/zephyr,galak/zephyr,punitvara/zephyr,kraj/zephyr,kraj/zephyr,GiulianoFranchetto/zephyr,kraj/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,GiulianoFranchetto/zephyr,mbolivar/zephyr,punitvara/zephyr