dfu: add backend for MTD device

Add DFU backend for MTD device: allow to read
and write on all MTD device (NAND, SPI-NOR,
SPI-NAND,...)

For example :
> set dfu_alt_info "nand_raw raw 0x0 0x100000"
> dfu 0 mtd nand0

This MTD backend provides the same level than dfu nand
backend for NAND and dfu sf backend for SPI-NOR;
So it can replace booth of them but it also
add support of spi-nand.

> set dfu_alt_info "nand_raw raw 0x0 0x100000"
> dfu 0 mtd spi-nand0

The backend code is based on the "mtd" command
introduced by commit 5db66b3aee6f ("cmd: mtd:
add 'mtd' command")

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
diff --git a/drivers/dfu/Kconfig b/drivers/dfu/Kconfig
index 1cbec81..1e1dff3 100644
--- a/drivers/dfu/Kconfig
+++ b/drivers/dfu/Kconfig
@@ -54,5 +54,11 @@
 	  This option enables the support of "part" and "partubi" target in
 	  SPI flash DFU back end.
 
+config DFU_MTD
+	bool "MTD back end for DFU"
+	depends on MTD
+	help
+	  This option enables using DFU to read and write to on any MTD device.
+
 endif
 endmenu