Convert CONFIG_CMD_ZFS to Kconfig

This converts the following to Kconfig:
   CONFIG_CMD_ZFS

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 fbedf98..5719a61 100644
--- a/cmd/Kconfig
+++ b/cmd/Kconfig
@@ -1415,6 +1415,17 @@
 	  that device writes are sequential regardless of filesystem
 	  activity.
 
+config CMD_ZFS
+	bool "zfs - Access of ZFS filesystem"
+	help
+	  This provides commands to accessing a ZFS filesystem, commonly used
+	  on Solaris systems. Two sub-commands are provided:
+
+	    zfsls - list files in a directory
+	    zfsload - load a file
+
+	  See doc/README.zfs for more details.
+
 endmenu
 
 menu "Debug commands"