board: Add Canaan KD233 board support

board: Add Canaan KD233 board support

Add a buildroot configuration file to build a minimal Linux environment
for the Canaan KD233 board.

The configuration file is canaan_kd233_defconfig. It builds a bootable
kernel image with an embedded initramfs root file system. The image
built can be flashed to the board as is and does not require a boot
loader. This configuration uses the tiny busybox configuration defined
in board/canaan/k210-soc/busybox-tiny.config.

U-Boot currently does not support this board, making it impossible to
boot the kernel after loading it from the SD card. However, the SD card
is usable from Linux once booted using the canaan_kd233_defconfig
configuration.

The configuration also enable the kflash and pyserial-miniterm host
tools for flashing image files to the board and opening a terminal
console.

The readme.txt file documents how to build and boot the Canaan KD233
board with this configuration.

Signed-off-by: Damien Le Moal <[email protected]>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <[email protected]>