Completely drop bts_log_init(), call osmo_init_logging2() directly instead: all
callers of bts_log_init() passed NULL as category string, so all it ever did
was call osmo_init_logging(). The bts_log_info is already declared in the .h.
Here and there also define a proper talloc root context instead of using NULL.
Change-Id: Ic049f77bef74123b95350bcae182a468e0086b9c