jakogut

Balbes150, I've got Linux 4.12.0-rc5 running on an Amlogic S905X based P212 device with your Arch image.
 
Next is the Mali kernel driver.

1. Button makes the built-in bootloader (u-boot) in the first place to look for on an external storage medium a file (aml_autoscript). If it is, go to it aml_autoscript. If there is no such file, the loader starts the built-in recovery mode. In theory, you could write your own version aml_autoscript that will load from external media new u-boot and will give him control. But it requires a good knowledge of the structure of u-boot (source code for your specific model) and the availability of special commands for the complete transfer of control the new u-boot on an external drive.
2. If you need to boot from external media (without making any changes multi-boot) you can rename the script s905_autoscript in aml_autoscript and pressing the button will immediately run the script to start the system from external media. In this embodiment, for system start up from external media will need to click when you enable this button.