We should see to customize echo mmc0 > /sys/class/leds/bananapi:green:usr/trigger
But maybe we can just add it in howto
This is what I put in the etc overlay directory to do. Before creating the image using RBF, create any files required in the new root's /etc directory in the etc overlay (default ./etc). Assuming that, in your case, your command needs to be run on every boot, in can go in ./etc/rc.d/rc.local.
But remember that the etc overlay is force copied to the new root, overwriting existing files. Hope this helps.
Regards Mandar Joshi