README: document DFU mode

Add instructions for how to use MCUboot for USB Device Firmware Upgrade
(DFU) mode.

Signed-off-by: Henrik Brix Andersen <[email protected]>

README.md

@@ -38,6 +38,7 @@ The CANnectivity firmware supports the following features, some of which depend
 - CAN bus error reporting
 - Automatic gs_usb driver loading under Linux using custom [udev rules](99-cannectivity.rules)
 - Automatic WinUSB driver installation under Microsoft Windows 8.1 and newer
+- USB Device Firmware Upgrade (DFU) mode
 
 ## Hardware Requirements
 
@@ -98,6 +99,38 @@ provided. These can be selected by setting either `FILE_SUFFIX=usbd_next` or
 
 After building, the firmware can be flashed to the board by running the `west flash` command.
 
+## USB Device Firmware Upgrade (DFU) Mode
+
+CANnectivity supports USB Device Firmware Upgrade
+([DFU](https://docs.zephyrproject.org/latest/services/device_mgmt/dfu.html)) via the
+[MCUboot](https://www.trustedfirmware.org/projects/mcuboot/) bootloader. This is intended for use
+with boards without an on-board programmer.
+
+To build CANnectivity with MCUboot integration for USB DFU use
+[sysbuild](https://docs.zephyrproject.org/latest/build/sysbuild/index.html) with the
+`sysbuild-dfu.conf` configuration file when building for your board (here `frdm_k64f`):
+
+```shell
+west build -b frdm_k64f/mk64f12  --sysbuild ../custom/cannectivity/app/ -- -DSB_CONF_FILE=sysbuild-dfu.conf
+```
+
+After building, MCUboot and the CANnectivity firmware can be flashed to the board by running the
+`west flash` command.
+
+Subsequent CANnectivity firmware updates can be applied via USB DFU. In order to do so, the board
+must first be booted into USB DFU mode. If your board has a dedicated DFU button (identified by the
+`mcuboot-button0` devicetree alias) press and hold it for 5 seconds or press and hold the button
+while powering up the board. If your board has a DFU LED (identified by the `mcuboot-led0`
+devicetree alias), the LED will flash while the DFU button is being held and change to constant on
+once DFU mode is activated. Refer to your board documentation for further details.
+
+Once in DFU mode, the CANnectivity firmware can be updated using
+[dfu-util](https://dfu-util.sourceforge.net/):
+
+```shell
+dfu-util -a 1 build/app/zephyr/zephyr.signed.bin.dfu
+```
+
 ## CANnectivity as a Zephyr Module
 
 The CANnectivity firmware repository is a [Zephyr