Convert CONFIG_CMD_YAFFS2 to Kconfig

This converts the following to Kconfig:
   CONFIG_CMD_YAFFS2

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>