[RFC] [WIP] 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?
I was editing `configure.in` to replace this string, but discovered https://github.com/xbmc/xbmc/blob/3c2b0ebb45c0e2bd475577c320c656f5569754d5/configure.in#L18. So may be really update `APP_NAME_LC` string to use existing function?