[RFC] [WIP] Rebranding update: Rename APP_HOME -> KODI_HOME

KODI_HOME should be more logical.
`XBMC_HOME` is used widely, but it's not very logical to set variable `APP_HOME` to run Kodi.
This PR fix it.
@FernetMenta @Memphiz @Montellese @topfs2 your thoughts?