bts_*: Add ready_for_pcu API

This is required to start osmo-pcu after osmo-bts is already setup and
activated. Otherwise osmo-pcu ends after connecting to socket with:
"pcu_l1_if.cpp:416 BTS not available"

Change-Id: I7209589f60bda63094336e417638906be5e273c4
6 files changed