Much simpler would be...Every Armbian image for the s802/s812 comes with it in the boot partition. When you burn the *.img to the SD card, turn on your box with the recovery button pressed in using the toothpick method. you do have a recovery button? the boot logo flashes once then let go of the button and should flash once more on its own. you will hear a faint piezo tone from the box when it flashes..but those are details. just watch for the logo flashing off and back on. and you're set.
Updated images Armbian 20170531. Changes to scripts activate the multi-boot and run system from external media. In the new version of the scripts allow to start the system from any USB socket (if the is connected other USB media). Back support "nodm", now in the images is used in common with the main branch Armbian authorization system. If you want you can install the package "lightdm" in version Mate-that-be to have a graphical authentication when you log into the system (just don't forget together with the installation of lightdm, remove the nodm package, otherwise they will interfere with each other). Other changes see the description of changes to main branch Armbian (replacement browser on Chromium, etc.).
New scripts activate the multi-boot and system start-up can be found here. You can download them and use for old images. To reactivate the multi-boot again. https://yadi.sk/d/cEPIULGW3Jgpee https://yadi.sk/d/QCGU0PIv3G5kEa
New script to activate the multi-boot on the Minix models H8x and H8x plus. https://yadi.sk/d/BU_pyfHW3JgpEP