doc: canaan: Add K230 CanMV board

Add description of compiling u-boot for K230 CanMV.

Since the vendor's u-boot-spl verifies u-boot header [1], it is
necessary to use the Python script from vendor to add the header to the
u-boot image.

[1] https://github.com/kendryte/k230_sdk/blob/v1.8/src/little/uboot/board/canaan/common/k230_board_common.h#L52

Signed-off-by: Junhui Liu <junhui.liu@pigmoral.tech>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>
3 files changed