Hi @tobin
If you add dtdebug=on to config.txt the early boot that happens in the VideoCore 4 GPU will be logged.
To see the log after Linux is booted, use vcdbg log msg.
Also see this “freshman” question thread in this forum.
(caveat, I’ve not (yet) used any SPI devices)
[edit]
I just now tried this on my Rpi3B+ with latest Nerves etc. and got the log ::
Failed to load overlay 'spi1-3cs'
brfs: File read: /mfs/sd/overlays/spi1-3cs.dtb
and sure enough, /boot/overlays does not contain any spi1*.dt?
Drop the right .dtbo in there and things might work 
[edit++]
To "drop the right .dtbo . . " you need to edit fwup.conf to make sure the spi1-3cs.dtbo file is copied in all the correct places.
