docs: rewrite HowToAddnewBoard.md
docs: rewrite HowToAddnewBoard.md

Previous is not enough to know how to add a new board.
This commit describes details.

Signed-off-by: sunghan-chang <c954e25285721d8faa8b1a5c1ae2ef5a35338d32@samsung.com>

sunghan-chang/TizenRT,sunghan-chang/TizenRT,junmin-kim/TizenRT,an4967/TizenRT,sunghan-chang/TizenRT,jeongarmy/TizenRT,jeongchanKim/TizenRT,an4967/TizenRT,chanijjani/TizenRT,pillip8282/TizenRT,Samsung/TizenRT,JeonginKim/TizenRT,davidfather/TizenRT,Samsung/TizenRT,davidfather/TizenRT,JeonginKim/TizenRT,an4967/TizenRT,jeongchanKim/TizenRT,davidfather/TizenRT,JeonginKim/TizenRT,jeongarmy/TizenRT,davidfather/TizenRT,HONGCHAEHEE/TizenRT,pillip8282/TizenRT,jsdosa/TizenRT,Samsung/TizenRT,an4967/TizenRT,JeonginKim/TizenRT,chanijjani/TizenRT,jeongchanKim/TizenRT,jsdosa/TizenRT,junmin-kim/TizenRT,jeongchanKim/TizenRT,davidfather/TizenRT,jeongarmy/TizenRT,jsdosa/TizenRT,Samsung/TizenRT,pillip8282/TizenRT,chanijjani/TizenRT,jsdosa/TizenRT,HONGCHAEHEE/TizenRT,sunghan-chang/TizenRT,an4967/TizenRT,junmin-kim/TizenRT,sunghan-chang/TizenRT,chanijjani/TizenRT,Samsung/TizenRT,Samsung/TizenRT,jsdosa/TizenRT,chanijjani/TizenRT,pillip8282/TizenRT,sunghan-chang/TizenRT,jsdosa/TizenRT,pillip8282/TizenRT,chanijjani/TizenRT,jeongchanKim/TizenRT,an4967/TizenRT,HONGCHAEHEE/TizenRT,JeonginKim/TizenRT,Samsung/TizenRT,jeongchanKim/TizenRT,junmin-kim/TizenRT,jeongarmy/TizenRT,HONGCHAEHEE/TizenRT,pillip8282/TizenRT,sunghan-chang/TizenRT,jeongchanKim/TizenRT,JeonginKim/TizenRT,davidfather/TizenRT,pillip8282/TizenRT,chanijjani/TizenRT,HONGCHAEHEE/TizenRT,junmin-kim/TizenRT,junmin-kim/TizenRT,HONGCHAEHEE/TizenRT,jeongarmy/TizenRT,an4967/TizenRT,junmin-kim/TizenRT,jeongarmy/TizenRT,jsdosa/TizenRT,davidfather/TizenRT,jeongarmy/TizenRT