Failure to install on MacBook Pro Retina early 2015

The MacBookPro uses UEFI and will only boot from a USB stick for the
install.
After using the Apple magic 'alt' key to boot from the USB stick I get
this:-
probing pc0 mem [list of numbers]
disk: hdo* hd1*
>> OpenBSD /amd64 BOOTX64 3.62
open (hd0a: /etc/boot.conf: Invalid argument
failed (22) will try /bsd

Using 'set' and 'help' doesn't yield any useful info to find the
boot.conf file.
ls gives hd0a Invalid Argument
ls hd0a gives hd0a:hd0a Invalid Argument

set shows the device as hd0.
hd0a is definitely the USB stick.

Is this install doomed to fail or am I missing some secret info that
rummaging through the boot(8)
boot_config(8) and boot_configamd64(8) man pages has failed to find?

The MacBook Pro is currently running Devuan Chimaera linux to avoid
systemd...

Thankyou,

DaveT