osxbundle: fix cascading config loading

mpv does 'cascading' configs by overriding options as the config become more
important (bundle -> system level -> user level).

Unfortunately mpv also loads two sets of configs files one after the other [1].
First it looks for 'config', then for 'mpv.conf'. For this reason a mpv.conf in
ANY location will override ANY config files named config (even if the mpv.conf
is in a system path and config in a user path).

[1]: Relevant code in player/configfiles.c

    load_all_cfgfiles(mpctx, section, "config");
    load_all_cfgfiles(mpctx, section, "mpv.conf");

Fixes: #1361