config: add CERBERO_HOST_SOURCES env variable
config: add CERBERO_HOST_SOURCES env variable

In the daily usage of Cerbero's shell,
it's very convenient have CERBERO_HOST_SOURCES
pointing to current sources directory,
to jump fast there instead of browsing from CERBERO_PREFIX.

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