cerbero: Call load_platform_config() once correctly
cerbero: Call load_platform_config() once correctly

This reverts 5be60e8bc69725616bd54ee1d2983afb2db2469a. The mistake in
that commit was that `self._load_platform_config()` ends up not being
called at all when building for a universal platform, which causes,
f.ex., bootstrap to fail.

See: https://gitlab.freedesktop.org/gstreamer/cerbero/merge_requests/385#note_398864

The new way of not calling those functions twice is clearer.

GStreamer/cerbero,nirbheek/cerbero,GStreamer/cerbero,nirbheek/cerbero,nirbheek/cerbero,GStreamer/cerbero,GStreamer/cerbero,nirbheek/cerbero,GStreamer/cerbero