pico-imx7d: README: Adjust the binary name after DM conversion

After the conversion to DM the U-Boot binary is called u-boot-dtb.imx,
so fix the README file accordingly.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Acked-by: Joris Offouga <offougajoris@gmail.com>
diff --git a/board/technexion/pico-imx7d/README b/board/technexion/pico-imx7d/README
index f21d830..6aa0d25 100644
--- a/board/technexion/pico-imx7d/README
+++ b/board/technexion/pico-imx7d/README
@@ -11,7 +11,7 @@
 $ make pico-imx7d_defconfig
 $ make
 
-This generates the SPL and u-boot.img binaries.
+This generates the SPL and u-boot-dtb.img binaries.
 
 1. Loading U-Boot via USB Serial Download Protocol
 
@@ -32,15 +32,15 @@
 
 Open a terminal program such as minicom.
 
-Copy SPL and u-boot.img to the imx_usb_loader folder.
+Copy SPL and u-boot-dtb.img to the imx_usb_loader folder.
 
 Load the SPL binary via USB:
 
 $ sudo ./imx_usb SPL
 
-Load the u-boot.img binary via USB:
+Load the u-boot-dtb.img binary via USB:
 
-$ sudo ./imx_usb u-boot.img
+$ sudo ./imx_usb u-boot-dtb.img
 
 Then U-Boot starts and its messages appear in the console program.
 
@@ -55,11 +55,11 @@
 
 => dfu 0 mmc 0
 
-Flash SPL and u-boot.img into the eMMC running the following commands on a PC:
+Flash SPL and u-boot-dtb.img into the eMMC running the following commands on a PC:
 
 $ sudo dfu-util -D SPL -a spl
 
-$ sudo dfu-util -D u-boot.img -a u-boot
+$ sudo dfu-util -D u-boot-dtb.img -a u-boot
 
 Remove power from the pico board.