fill-in build_system section

Signed-off-by: Michel-FK <michel.stempin@funkey-project.com>

docs/developer_guide/tutorials/build_system/write_image.md

@@ -0,0 +1,32 @@
+## How to Flash the Firmware to the SD Card
+
+You can copy the bootable `images/sdcard.img` onto an SD card using
+"dd":
+
+```bash
+$ sudo dd if=images/FunKey-sdcard-X.Y.Z.img of=/dev/sdX
+```
+<ins>Warning</ins>: Please make sure that */dev/sdX* device
+corresponds to your SD Card, otherwise you may wipe out one of your
+hard drive partitions!
+
+Alternatively, you can use the Balena-Etcher graphical tool to burn
+the image to the SD card safely and on any platform:
+
+https://www.balena.io/etcher/
+
+Once the SD card is burnt, insert it into the **FunKey S** console
+slot, and power it up. Your new system should come up now and start a
+console on the UART0 serial port and display the RetroFE game launcher
+on the graphical screen.
+
+## How to Update the FunKey S Firmware
+
+It is possible to update the **FunKey-S** over USB, please follow the
+steps described in the [Firmware Upgrade][1] section.
+
+[1]: /user_manual/tutorials/software/firmware_update/
+
+--8<--
+includes/glossary.md
+--8<--