trx_toolkit/fake_trx.py: refactor L12TRX -> TRX2L1 burst transformation
The burst transformation in BurstForwarder.forward_msg() used to be
done only once, so then the resulting message was distributed over
the list of connected (and active) transceivers.
This approach limits the path loss simulation capabilities, because
a reference to the same message is passed to FakeTRX.send_data_msg().
If one transceiver changes (or removes) the burst bits, the other
transceivers would not receive the original message.
Let's do the transformation individually for each transceiver,
so the original message will always remain unchanged.
Change-Id: Ia016a3a9bb6e9f17182a7168aa5a501ae9b9978b