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>
diff --git a/cmd/Kconfig b/cmd/Kconfig
index 3dfc94a..a5c557e 100644
--- a/cmd/Kconfig
+++ b/cmd/Kconfig
@@ -1369,6 +1369,17 @@
 	  scan the bus, reset the bus, read and write data and get information
 	  about devices.
 
+config CMD_YAFFS2
+	bool "yaffs2 - Access of YAFFS2 filesystem"
+	depends on YAFFS2
+	default y
+	help
+	  This provides commands for accessing a YAFFS2 filesystem. Yet
+	  Another Flash Filesystem 2 is a filesystem designed specifically
+	  for NAND flash. It incorporates bad-block management and ensures
+	  that device writes are sequential regardless of filesystem
+	  activity.
+
 endmenu
 
 menu "Debug commands"