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/fs/yaffs2/Kconfig b/fs/yaffs2/Kconfig
new file mode 100644
index 0000000..45ffdf6
--- /dev/null
+++ b/fs/yaffs2/Kconfig
@@ -0,0 +1,7 @@
+config YAFFS2
+	bool "YAFFS2 filesystem support"
+	help
+	  This provides access to YAFFS2 filesystems. 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.