Change APP_HOME -> KODI_HOME and other APP_* -> KODI_*

Extracted from #5560

This is the straight forward change from APP_* env variables to KODI_* env variables. As pointed out in the other PR the APP_* might conflict with other apps in the future.

I know this is not generic - but it wasn't in the initial rebrand aswell. Imo we should focus on getting helix out and therefore cleanup the multiple method mess afterwards (i think karlson2k is on the best way - but there is no need to do it for helix).
Why the change from ="foo" to ="". You added documentation and more code in the function for no reason afaicf.