You can get a system for installing additional software by:

1. plugging in a partitioned, ext4 formatted USB device
2. enabling Download Master on that USB device

The Download Master installation mounts a subdirectory on the USB device to opt/bin and it fetches a bunch of packages including ipkg, the same tool that manages packages on OpenWRT.

Entware is a repository of prebuilt packages that can be installed on the USB device.

wget -O - http://bin.entware.net/aarch64-k3.10/installer/generic.sh | sh

Using a standard init.d startup script system services can be launched on boot.