Changes

pcuif: add comment on TXT_IND/PCU_VERSION BTS initialization behavior

The PCU sends a TXT_IND with the PCU_VERSION as the first message
after the socket connection is established. The BTS/BSC uses those
messages to trigger some initialization (allocate BTS object inside
the PCU, send SI to the PCU).

Unfortunately the PCU will send the TXT_IND with the PCU_VERSION
always with to BTS 0. The 0 in this case is a hardcoded fake BTS
number (There is no BTS object allocated under this number in the
PCU at this point). However, the BTS/BSC will then do the
initialization process for BTS 0, which works by chance for the
BTS where the BTS number is always 0, in the case of the BSC there
may be multiple BTS configured, so there may be arbitrary BTS
numbers.

Let's fix this design error by agreeing that a TXT_IND that has the
type PCU_VERSION is always addressing the receiving process as a whole
and therefore the BTS number in the indication must be ignored.

Related: OS#6507
Change-Id: Ib3f340ae9450151549ce61d34b28253d499ae04a