trx_toolkit/fake_trx.py: handle multiple transceivers

trx_toolkit/fake_trx.py: handle multiple transceivers

Since Ice44e2b22566b3652ef6d43896055963b13ab185 is merged, the class
hierarchy has become much more flexible, so it's possible to create
multiple Transceiver / FakeTRX instances and distribute bursts using
a single instance of BurstForwarder.

This change introduces a new command line option, that can be used
to specify additional transceivers. Please note that fake_trx.py
still initializes a pair of BTS and BB transceivers by default.

The new option has the following format:

  --trx REMOTE_ADDR:BASE_PORT[/TRX_NUM]

Some examples for IPv4 and IPv6:

  --trx 127.0.0.1:5703
  --trx [2001:0db8:85a3:0000:0000:8a2e:0370:7334]:6703

If optional TRX_NUM > 0 is specified, e.g.:

  --trx 127.0.0.1:5700/1

then this transceiver is considered as a child of another one.
See I7e97b7f32dde7ab74779133e9d7504f1d0fce60c for details.

Change-Id: Ibff31fb3a958a714c828d0dea7e87d47f778fd80