This change makes it possible to configure l1sched related logging:
* l1sched_logging_init() allows to configure logging categories for
common and data messages (by default, DLGLOBAL is used);
* struct l1sched_cfg allows to configure a logging prefix to be
used in l1sched messges (by default, 'l1sched[0x%p] is used)'.
Let's use osmo_fsm_inst_name(trxcon->fi) as the logging prefix.
Change-Id: I26da1a506b02502a3a6a887533c35fb09c13c429
Related: OS#5599, OS#3761