This is a regression of the code compared to the existing
sysmoBTS code, where the L1 tells us whether its AGCH or
PCH. However, it was not used even in the old code, so
we can afford to simply put a #warning here.
This first part moves BCCH message primitives from osmo-bts-sysmo to common
part. A new file "common/l1sap.c" is introduced to implement handling of
layer 1 messages from/to BTS model.