Added Instructions to flash or update U-Boot

docs/software/flashing_u-boot.md

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+# Flashing U-Boot onto the SPI-NOR
+
+This guide provides detailed steps to flash U-Boot onto the SPI-NOR of the Kumquat board using the **sunxi-fel** tool. This process requires a host computer and the Kumquat board connected via USB.
+
+## Prerequisites
+
+Before starting, ensure you have the following:
+
+- **Host Computer Requirements:**
+  - A serial terminal application (e.g., Putty, Minicom).
+  - **sunxi-fel** installed on the host system, with the necessary udev rules or run as root.
+  - The new **nor-flash.img** file, either built yourself or downloaded from the releases page:
+    
+    [Download nor-flash.img](https://git.netcubesystems.at/NetCube-Systems-Austria/kumquat-buildroot-releases/releases)
+
+- **Hardware Requirements:**
+  - A USB-C console cable.
+  - A USB-C OTG cable (one cable is sufficient, but two are recommended to avoid swapping ports).
+
+## Step-by-Step Instructions
+
+### Step 1: Enter the Bootloader
+
+1. Connect the Kumquat board to your host computer using the USB-C console cable.
+2. Power on the Kumquat board and monitor the serial terminal.
+3. When prompted, press any key in the terminal to interrupt the boot process and enter the U-Boot bootloader.
+
+*Note: If there is no bootloader on the board, you can skip to Step 3.*
+
+### Step 2: Prepare the SPI-NOR Flash for FEL Mode
+
+1. In the U-Boot bootloader, run the following commands to erase the boot area of the SPI-NOR flash:
+
+   ```
+   sf probe
+   sf erase 0 0x1000
+   reset
+   ```
+
+2. The system will reboot. Since the boot area is erased, the SoC will automatically enter **FEL Mode**.
+
+### Step 3: Flash the SPI-NOR with the New U-Boot Image
+
+1. Connect the Kumquat board to your host computer using the USB-OTG port.
+2. On the host computer, run the following command to flash the **nor-flash.img** onto the SPI-NOR:
+
+   ```
+   sunxi-fel -p spiflash-write 0 nor-flash.img
+   ```
+
+3. Once flashing is complete, reboot the Kumquat board using:
+
+   ```
+   sunxi-fel wdreset
+   ```
+
+   The board should now boot into the new U-Boot.
+
+### Troubleshooting
+
+- If **sunxi-fel** reports no device or no access, try running the same command again. In some cases, it may take a few attempts to successfully communicate with the device.
+
+### Notes
+
+- Ensure the **sunxi-fel** tool is correctly installed on your host system. If running without udev rules, use `sudo` for all commands.
+- Always verify the integrity of the **nor-flash.img** file before flashing.
+
+By following these steps, you can successfully flash the U-Boot bootloader onto the SPI-NOR of the Kumquat board.
+