bsc: Allow to apply configuration for an individual BTS

This will drop a specific IP based BTS. It will lead to a
re-connect of the BTS and the new settings will be applied
then.

Fixes: SYS#737
bsc: Allow to set the call-identity

Allow to set the cell-identity through the control interface
and add a small test for it.

Related: SYS#739
bts: Store the bcch_change_mark in the bts structure

Store the BCCH change mark inside the BTS structure. This will
allow us increment the number and re-generate the SIs.

Related: SYS#739
bsc: Allow to generate new system information online

Increase the bcch_change_mark and generate a new copy of the
system information. Make the method public, add a small test
case. Manually verified using the FakeBTS. I don't know if
the MS will re-read these SIs.

Related: SYS#739
bsc: Add ctrl command to set the TRX ARFCN