dfu: add DFU virtual backend

Add a virtual DFU backend to allow board specific read and write
(for OTP update for example).

Acked-by: Lukasz Majewski <lukma@denx.de>
Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
diff --git a/drivers/dfu/dfu.c b/drivers/dfu/dfu.c
index 4f4a07b7..2697235 100644
--- a/drivers/dfu/dfu.c
+++ b/drivers/dfu/dfu.c
@@ -474,6 +474,9 @@
 	} else if (strcmp(interface, "sf") == 0) {
 		if (dfu_fill_entity_sf(dfu, devstr, s))
 			return -1;
+	} else if (strcmp(interface, "virt") == 0) {
+		if (dfu_fill_entity_virt(dfu, devstr, s))
+			return -1;
 	} else {
 		printf("%s: Device %s not (yet) supported!\n",
 		       __func__,  interface);
@@ -569,7 +572,7 @@
 const char *dfu_get_dev_type(enum dfu_device_type t)
 {
 	const char *const dev_t[] = {NULL, "eMMC", "OneNAND", "NAND", "RAM",
-				     "SF", "MTD"};
+				     "SF", "MTD", "VIRT"};
 	return dev_t[t];
 }