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/Kconfig b/fs/Kconfig
index e6438ad..e6803ac 100644
--- a/fs/Kconfig
+++ b/fs/Kconfig
@@ -18,4 +18,6 @@
 
 source "fs/cramfs/Kconfig"
 
+source "fs/yaffs2/Kconfig"
+
 endmenu