board: Add Sipeed MAIXDUINO board support

board: Add Sipeed MAIXDUINO board support

Add two buildroot configuration files to build a minimal Linux
environment for the Sipeed MAIXDUINO board. The configurations are:
* sipeed_maixduino_defconfig: Build 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.
* sipeed_maixduino_sdcard_defconfig: Build a kernel image with a root
  file system on the SD card and using U-Boot as the boot loader. This
  uses the default busybox minimal configuration.

Both configurations 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 Sipeed MAIXDUINO
board with these configurations.

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