pico-imx7d: Update the README file

Update the README file to take into accound the switch to SPL.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Fabio Berton <fabio.berton@ossystems.com.br>
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>

board/technexion/pico-imx7d/README

@@ -11,7 +11,13 @@ $ make mrproper
 $ make pico-imx7d_defconfig
 $ make
 
-This generates the U-Boot binary called u-boot.imx.
+This generates the SPL and u-boot.img binaries.
+
+1. Loading U-Boot via USB Serial Download Protocol
+
+Note: This method is convenient for development purposes.
+If the eMMC has already a U-Boot flashed with DFU support then
+the user can go to step 2 below in order to update U-Boot.
 
 Put pico board in USB download mode (refer to the PICO-iMX7D Quick Start Guide
 page 3)
@@ -22,11 +28,15 @@ Connect a USB cable between the OTG pico port and the host PC.
 
 Open a terminal program such as minicom.
 
-Copy u-boot.imx to the imx_usb_loader folder.
+Copy SPL and u-boot.img to the imx_usb_loader folder.
+
+Load the SPL binary via USB:
 
-Load u-boot.imx via USB:
+$ sudo ./imx_usb SPL
 
-$ sudo ./imx_usb u-boot.imx
+Load the u-boot.img binary via USB:
+
+$ sudo ./imx_usb u-boot.img
 
 Then U-Boot starts and its messages appear in the console program.
 
@@ -35,16 +45,16 @@ Use the default environment variables:
 => env default -f -a
 => saveenv
 
+2. Flashing U-Boot into the eMMC
+
 Run the DFU agent so we can flash the new images using dfu-util tool:
 
 => dfu 0 mmc 0
 
-Flash SPL into the eMMC:
+Flash SPL and u-boot.img into the eMMC running the following commands on a PC:
 
 $ 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.