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/Makefile b/drivers/dfu/Makefile
index ebb119f..0d7925c 100644
--- a/drivers/dfu/Makefile
+++ b/drivers/dfu/Makefile
@@ -10,3 +10,4 @@
 obj-$(CONFIG_$(SPL_)DFU_RAM) += dfu_ram.o
 obj-$(CONFIG_$(SPL_)DFU_SF) += dfu_sf.o
 obj-$(CONFIG_$(SPL_)DFU_TFTP) += dfu_tftp.o
+obj-$(CONFIG_$(SPL_)DFU_VIRT) += dfu_virt.o