The LVGL Example App `lvgldemo` currently requires 1 argument: the name of the demo to show.
```bash
lvgldemo widgets
```
In this PR, we propose to make the argument optional if there is only one demo configured. This will enable PINE64 PinePhone to boot straight into `lvgldemo` and auto-start the LVGL App, without entering any `nsh` commands.
### Modified Files
`examples/lvgldemo/lvgldemo.c`: If no arguments are specified and only 1 demo exists, select the demo
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