From FAQ thread:
* How to make Kodi launch on startup? For firmware 3.0.2 and later, go to apps menu from home screen. Using remote, press menu key when hovering over app. You will see an option for 'default start'. Alternatively, there are several free apps in Play Store designed to do this. We recommend Autostart and Stay!: https://play.google.com/store/apps/details?id=com.atasoglou.autostartandstay&hl=en_GB