As root open firewall for incoming connections:

su
iptables -P INPUT ACCEPT

Copy the syncthing binary from the Linux/ARM build onto the SD CARD.

Move into a folder that doesn't have the noexec flag (/data).
busybox cp /extsd/syncthing /data/

Become the system user.

su system

Set the HOME environment variable.

export HOME=/extsd

Launch Syncthing
/data/syncthing