I can turn off the system light by running "echo 0 > /sys/class/leds/white:system/brightness".
However, I was unable to control the big button:
opkg install kmod-button-hotplug
uci add system button
uci set system.@button[-1].button="led"
uci set system.@button[-1].action="pressed"
uci set system.@button[-1].handler="echo 1 > /sys/class/leds/white:system/brightness"
uci add system button
uci set system.@button[-1].button="led"
uci set system.@button[-1].action="released"
uci set system.@button[-1].handler="echo 0 > /sys/class/leds/white:system/brightness"
uci commit system
Did I miss anything?