Added more information on SPI page

* Added screenshot of nand-sata-install
* Added more info to use U-Boot image other than from Armbian package
* Added instruction to write U-Boot under U-Boot

docs/spi.md

@@ -40,19 +40,62 @@ mmcblk0     179:0    0 14.9G  0 disk
 └─mmcblk0p1 179:1    0 14.8G  0 part /
 ```
 
-4) Run **nand-sata-install** utility
+4) Get the U-Boot SPI image
+
+   ***A. Install/Update U-Boot package from Armbian***
+
+```
+sudo apt-get update
+sudo apt-get -y install linux-u-boot-helios4-next
+```
+
+   ***B. Download the Prebuilt Image from Wiki***
+
+   *U-Boot 2018.11*
+
+```
+wget https://wiki.kobol.io/files/uboot/u-boot-armbian-2018.11-spi.bin
+sudo mv u-boot-armbian-2018.11-spi.bin /usr/lib/linux-u-boot-next-helios4_*/u-boot.flash
+```
+
+   *Marvell U-Boot 2013.01*
+
+```
+wget https://wiki.kobol.io/files/uboot/u-boot-armbian-2013.01-spi.bin
+sudo mv u-boot-armbian-2013.01-spi.bin /usr/lib/linux-u-boot-next-helios4_*/u-boot.flash
+```
+
+   ***C. Custom U-Boot SPI Image***
+
+   Upload the U-Boot SPI binary that you built on your PC to Helios4 and rename it as **u-boot.flash**.
+   For example, if the Helios4 IP address is 10.10.0.10 the command would be:
+
+```
+scp u-boot-spl.kwb root@10.10.0.10:/usr/lib/linux-u-boot-next-helios4_*/u-boot.flash
+```
+
+!!! note
+    Replace **u-boot-spl.kwb** with **u-boot-a38x-*-spi.bin** if you are using Marvell U-Boot 2013.01
+
+5) Run **nand-sata-install** utility
 
 ```
 sudo nand-sata-install
 ```
 
-5) Select option **Install/Update the bootloader on SPI Flash**
+6) Select option **Install/Update the bootloader on SPI Flash**
 
-6) If you want to take the opportunity to move your RootFS to another device, jump to this [section](#moving-rootfs-to-other-device). Otherwise you may disable spi_workaround in /boot/armbianEnv.txt
+![Install Bootloader menu](/img/spi/armbian_install_u-boot_menu.png)
+
+And confirm the operation
+
+![Bootloader write confirmation](/img/spi/armbian_install_u-boot_write_confirmation.png)
+
+7) If you want to take the opportunity to move your RootFS to another device, jump to this [section](#moving-rootfs-to-other-device). Otherwise you may disable spi_workaround in /boot/armbianEnv.txt
 
 `spi_workaround=off`
 
-7) Set DIP switches **SW1** to SPI Boot and reboot the system.
+8) Set DIP switches **SW1** to SPI Boot and reboot the system.
 
 ![Boot from SPI](/img/spi/dipswitch_boot_spinor.png)
 
@@ -74,7 +117,7 @@ sudo wget https://wiki.kobol.io/files/uboot/boot_spi_en.scr -O /boot/boot_spi_en
 3) Switch to Helios4 serial console, then reboot the system
 
 ```
-sudo Reboot
+sudo reboot
 ```
 
 4) Press any key to cancel the U-Boot autoboot and execute these commands
@@ -113,6 +156,37 @@ BootROM - 1.73
 Booting from SPI flash
 ```
 
+### Under U-Boot
+
+!!! info
+    You will need to access to Helios4 via Serial Console. Please refer to [Install](/install/#step-4-connect-to-helios4-serial-console) page for instructions.
+
+1) Upload the U-Boot SPI binary to /boot folder on Helios4 and rename it as **u-boot-spi.bin**.
+
+For example, if the Helios4 IP address is 10.10.0.10 the command would be:
+
+```
+scp u-boot-spl.kwb root@10.10.0.10:/boot/u-boot-spi.bin
+```
+
+2) Switch to Helios4 serial console, login then reboot the system
+
+```
+sudo reboot
+```
+
+3) Press any key to cancel the U-Boot autoboot and execute these commands
+
+```
+load mmc 0:1 ${loaddaddr} /boot/u-boot-spi.bin
+sf probe
+sf erase 0 100000
+sf write ${loadaddr} 0 ${filesize}
+reset
+```
+
+4) Helios4 would reboot and boot to existing operating system.
+
 ## Set Up U-Boot Environment
 
 !!! important
@@ -159,6 +233,10 @@ sudo nand-sata-install
 
 3) Select option **Boot from SPI  - system on SATA, USB or NVMe**
 
+![Move RootFS menu](/img/spi/armbian_move_rfs_menu.png)
+
+And follow the on screen instructions.
+
 4) When RootFS migration is done, disable spi_workaround.
 
 5) Set DIP switches **SW1** to SPI Boot and reboot the system.