[RFC] Rebranding update: Rename APP_HOME -> KODI_HOME

After #5216 Kodi variable for home directory is called `APP_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.
And `APP_HOME` can be set by some other app/tool. If Kodi will be run on such system, it will not find libs and other files.
This PR fix it.

@FernetMenta @Memphiz @Montellese @topfs2 your thoughts?