I've definitely set it in sdcard's extlinux/extlinux.conf and install-2018.sh should be copying files from sdcard. And if uboot install is ok it should try sdcard first (this how it works with stock uboot). But in my case LED screen stayed off, no boot with sdcard inserted.
Maybe I'll give it another try later, now it is much easier when I know that it can be revived any time.
After reflashing with stock image and u-boot box boots without sdcard inserted into system installed with install.sh. No need to install u-boot 2018.
what I did on VIM2 was
[when booted Armbian_5.64_Aml-s9xxx_Ubuntu_bionic_default_4.19.0-rc7_desktop_20181019 from SD]:
- in /boot/ copy s912-uboot.img to uboot.img
- in /root/ run install-2018.sh
perhaps not for faint of heart but it worked for me.