PHY: add parameter end_of_burst_delay for TDD

end_of_burst_delay is used to stop TX only "after a while".
If we stop right after effective signal, with USRP B210 and
B200mini, we observe a high EVM on the S subframe (on the
PSS).
A value of 400 (for 30.72MHz) solves this issue. This is
the default.

This default value can be changed in the configuration file.
For example:

RUs = (
    {
       local_rf       = "yes"
         nb_tx          = 1
         nb_rx          = 1
         att_tx         = 20
         att_rx         = 0;
         bands          = [7];
         max_pdschReferenceSignalPower = -27;
         max_rxgain                    = 105;
         eNB_instances  = [0];
         sf_extension = 312;
         end_of_burst_delay = 200;
    }
);

Here we would set a value of 200.

The value to put in the configuration file is for
30.72MHz. The value is scaled accordingly at runtime
(thus only one value to set for every RB configuration,
25, 50 or 100, leading to less problems when adapting
configuration files).

This option is for experts and should not be changed
randomly.