This suggests I should be able to do a DT overlay with
sudo enable_dtoverlay spi0 spi@ff190000 okay
but the enable_dtoverlay command is not found. BTW I'm a maker, not a developer, so if it seems I'm floundering here, I am :-) It would be really sweet if the armbian-config utility included an "interfaces" option similar to what the RPi has, where you can simply toggle GPIO, SPI, i2C, etc on and off.