Is this just for convenience? This just adds the correct installer script into the image, but doesn't actually install it... so couldn't someone just SSH in and do same commands?

Might be nice to instead emit a script that downloads the current script and then calls down to that, so that it's getting the current at actual install time not at image build time

For example add entware-install.sh that looks like:

#!/bin/bash
set -e

wget -O $(SYSDRV_DIR_OUT_ROOTFS)/usr/sbin/entware-install-generic.sh https://bin.entware.net/armv7sf-k3.2/installer/generic.sh
chmod +x $(SYSDRV_DIR_OUT_ROOTFS)/usr/sbin/entware-install-generic.sh
$(SYSDRV_DIR_OUT_ROOTFS)/usr/sbin/entware-install-generic.sh
# rm $(SYSDRV_DIR_OUT_ROOTFS)/usr/sbin/entware-install-generic.sh

I took the idea from http://freshtomato.org/ where they give the user the choice to install Entware or not. Most users won't bother - but people like me wouldn't mind actually running the script (and then after installing packages anyway we'd need to modify many things...)