Docs: Add how to add a new board

This shows how to port new architecture.

docs/HowToAddnewBoard.md

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+# How to add(port) a new board
+
+Changes in *[arch](#arch)* and *[configs](#configs)* folders are mandantory to support a new board.  
+Adding static library or expanding driver will be provided later.
+
+## Arch
+
+Changes in *arch* directory for a new board includes chip architecture and BSP for board.  
+Currently, Tizen RT supports only ARM architecture.  
+But if new board is another architecture, new architecture folder should be added in this path.
+
+### BSP
+
+There are three kinds of folders, core, chip architecture and board.  
+For example, armv7-r is a core architecture, s5j is a chip architecture and artik053 is a board.  
+
+At least, new board folder should be added.
+
+### Kconfig
+
+Modifications on below two Kconfigs show new board configurations on menuconfig.
+
+#### Chip
+Modify *os/arch/arm/Kconfig* file.
+
+1. Add new configurations as belows.
+```
+choice
+	prompt "ARM chip selection"
+
+config ARCH_CHIP_<CHIP_NAME>
+	bool "XXX"
+	select ARCH_HAVE_YYY
+endchoice
+
+config ARCH_CHIP
+	string
+	default "<CHIP_NAME>"       if ARCH_CHIP_<CHIP_NAME>
+```
+
+2. Include Kconfig as belows.
+```
+if ARCH_CHIP_<CHIP_NAME>
+source arch/arm/src/<CHIP_NAME>/Kconfig
+endif
+```
+
+#### Board
+Modify *os/arch/Kconfig.board* file.
+
+1. Add new configurations as belows.
+```
+choice
+	prompt "Select target board"
+
+config ARCH_BOARD_<BOARD_NAME>
+	bool "XXX Starter Kit"
+	depends on ARCH_CHIP_<CHIP_NAME>
+	select YYY
+endchoice
+
+config ARCH_BOARD
+	string
+	default "<BOARD_NAME>"           if ARCH_BOARD_<BOARD_NAME>
+```
+
+2. Include Kconfig as belows.
+```
+comment "Board-Specific Options"
+
+if ARCH_BOARD_<BOARD_NAME>
+source arch/arm/src/<BOARD_NAME>/Kconfig
+endif
+```
+
+## Configs
+
+Changes in *configs* folder includes build options and flash operations.
+
+1. Add a new folder with board name.
+```
+mkdir build/configs/<board-name>
+```
+
+2. Add the *script* folder and add the linker script file.
+```
+mkdir build/configs/<board-name>/scripts
+```
+
+3. Add a new folder with representing main program name under \<board-name\> folder.
+```
+mkdir build/configs/<board-name>/<program-name>
+```
+
+4. Put *defconfig* and *Make.defs* inside \<program-name\>.
+
+### defconfig
+
+This is a configurations set of representing folder name.  
+This is moved to .config at executing configure.sh and used to build Tizen RT.
+
+### Make.defs
+
+This is a build options set like setting of including path, ARM build options, linker script name and so on.  
+There is the DOWNLOAD definition to support "make download xx" command too.
+