fake_trx/ctrl_if_(bb|bts).py: add FAKE_TOA command
FAKE_TOA is an auxilary CTRL command, which may be used to update
the ToA (Timing of Arrival) value of forwarded bursts at runtime.
This is useful for testing the measurement processing
code in OsmoBTS.
The command is implemented for both BTS and BB CTRL interfaces
in two absolute and relative forms:
CMD FAKE_TOA <BASE> <THRESH>
CMD FAKE_TOA <+-BASE_DELTA>
The first form overwrites both ToA value and its treshold.
The second one is relative, and applies a delta
to the current ToA value.
The command affects Downlink bursts if sent on BTS CTRL
interface, and Uplink bursts if sent on the BB CTRL.
Change-Id: Ia23becec4104d47e7b22350db67b8834d6f1ad1b