If I could make UI mockups on the fly, I would, but something on the top of my mind:
- New update available message on LuCI, button is pressed to begin update process.
- Pop up window in the page (similar to attended sysupgrade) appears.
- It grabs a list of all installed packages (along with requested build and latest version number) and generates a file that the user is instructed to download with a button, and to run the 'Local OpenWrt Image Builder' tool (with link to download the tool if they haven't got it yet).
- User opens tool, and drag and drops the file into the tool.
- The tool lists a confirmation that it will download version X.X.X and include the following packages: x, y, z.
- User confirms by clicking 'Build image'.
- Image is built by downloading standard image and packages from OpenWrt.
- Image is ready, and is located in their Downloads folder (or whatever the default location will be).
- User goes back to LuCI and drags image to the upload section on the pop up window to flash and complete.