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To flash the board, first boot into recovery by stopping the U-Boot countdown, then enter => run recovery. Once in recovery, enter root as the username (no password), and set your network:

ip link set eth3 up
ip addr add 10.0.0.69/24 dev eth3
ip route add default via 10.0.0.1 dev eth3

(change IPs and interfaces to reflect your network of course)

Once you can ping an external host, run:

curl -kO https://opnsense.mono.si/experimental/OPNsense-[timestamp]-arm-aarch64-GATEWAY.img.gz | gunzip | dd of=/dev/mmcblk0 bs=1M

Once it's done flashing: $ reboot

Full readme and build process here