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/Kconfig b/drivers/dfu/Kconfig
index 1e1dff3..9fe5bc0 100644
--- a/drivers/dfu/Kconfig
+++ b/drivers/dfu/Kconfig
@@ -60,5 +60,12 @@
 	help
 	  This option enables using DFU to read and write to on any MTD device.
 
+config DFU_VIRT
+	bool "VIRTUAL flash back end for DFU"
+	help
+	  This option enables using DFU to read and write to VIRTUAL device
+	  used at board level to manage specific behavior
+	  (OTP update for example).
+
 endif
 endmenu