CBCH messages not visible in GSMTAP
When the GSMTAP CBCH SAPI is enabled in the VTY, OsmoBTS still doesn't log CBCH messages.
CBCH: Move processing via L1SAP
for some historical reason, CBCH handling was not using the normal
L1SAP boundary. Let's change that and traverse L1SAP just like for
e.g. BCCH which is quite similar to CBCH handling.
This also has the added benefit of logging CBCH via GSMTAP.