virtio: cmd: Add virtio command for virtio devices

Add 'virtio' command in U-Boot command line.

Signed-off-by: Tuomas Tynkkynen <tuomas.tynkkynen@iki.fi>
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/cmd/Kconfig b/cmd/Kconfig
index ad14c9c..d609f9d 100644
--- a/cmd/Kconfig
+++ b/cmd/Kconfig
@@ -1065,6 +1065,13 @@
 	help
 	  USB mass storage support
 
+config CMD_VIRTIO
+	bool "virtio"
+	depends on VIRTIO
+	default y if VIRTIO
+	help
+	  VirtIO block device support
+
 config CMD_AXI
 	bool "axi"
 	depends on AXI