pico-imx7d: README: Use dfu-util to flash U-Boot

The DFU allows a more user friendly use as the details where the
bootloader is installed are abstracted.
Signed-off-by: NOtavio Salvador <otavio@ossystems.com.br>

board/technexion/pico-imx7d/README

@@ -35,12 +35,17 @@ Use the default environment variables:
 => env default -f -a
 => saveenv
 
-Run the UMS command:
-=> ums 0 mmc 0
+Run the DFU agent so we can flash the new images using dfu-util tool:
 
-Transfer u-boot.imx to be flashed into the eMMC:
+=> dfu 0 mmc 0
 
-$ sudo dd if=u-boot.imx of=/dev/sdX bs=1K seek=1; sync
+Flash SPL into the eMMC:
+
+$ sudo dfu-util -D SPL -a spl
+
+Flash u-boot.img into the eMMC:
+
+$ sudo dfu-util -D u-boot.img -a u-boot
 
 Remove power from the pico board.