unified/hardkernel/firmware-update.md: Add

Signed-off-by: Filip Gołaś <[email protected]>

docs/unified/hardkernel/firmware-update.md

@@ -1 +1,43 @@
-firmware update
+# Firmware update
+
+The following documentation describes the process of Dasharo open-source
+firmware update. If your device is currently flashed with the proprietary
+firmware please refer to the [Initial deployment](initial-deployment.md)
+documentation.
+
+The update process may be different, depending on the currently installed
+Dasharo firmware version.
+
+For simplicity of the process we recommend using
+[Dasharo Tools Suite](../../dasharo-tools-suite/overview.md).
+
+Before starting the update procedure be sure to disable Dasharo BIOS Boot
+medium lock and Secure Boot:
+
+1. Power on the device.
+2. While the device is booting, hold the `DELETE` key to enter the UEFI Setup
+    Menu.
+3. Enter the `Dasharo System Features` menu using the arrow keys and Enter.
+4. Enter the [Dasharo Security Options](https://docs.dasharo.com/dasharo-menu-docs/dasharo-system-features/#dasharo-security-options)
+    submenu.
+5. Verify the state of the `Lock the BIOS boot medium` option - if the option
+    is chosen, press `Space` and then `F10` to save the changes.
+6. Go back to the main menu using the `ESC` key.
+7. Enter the `Device Manager` menu.
+8. Enter the [Secure Boot Configuration](https://docs.dasharo.com/dasharo-menu-docs/device-manager/#secure-boot-configuration)
+    submenu.
+9. Verify that the `Current Secure Boot State` field says Disabled - if not,
+    unselect the `Attempt Secure Boot` option below then press `F10` to save
+    the changes.
+10. Reboot the device to properly apply the changes.
+
+The settings of all the above options can be restored after a firmware
+update.
+
+=== "ODROID H4+"
+
+    ## Updating Dasharo
+
+    ```bash
+    flashrom -p internal -w hardkernel_odroid_h4_v<version>.rom --fmap -i bios
+    ```