bts: Add some simple dependency between different BTS

E.g. for the sysmoBTS2050 we have the requirement that the first
board connects before the second due clocking. The easiest point
to enforce this is the BSC. Add a simple bitmask based system to
allow to express dependencies for IP based systems.
bts: When one link drops.. check what needs to be dropped

In case a BTS is dropped, iterate over the list of BTS and check
if a dependency is now missing and then drop the BTS. This check
could lead to check of 256*256 checks (e.g. all BTS on each other
in the chain and the master is being dropped). The performance
aspect of it doesn't matter for our usecase. We expect to have
pairs of BTS right now.
ho: Make sure the timer is always stopped

In case of a ho_chan_activ_nack (sent due another bug inside
both sysmobts and openbsc) the code would not stop the timer
but free the datastructure. This can lead to a clear segfault
when the timer has expired. Create a "free" function which is
responsible to detach the handover structure, stop the timer
(which is idempotent) and free the structure.
ho: Copy the multirate config to the new lchan

The new lchan will be in speech mode already but for AMR we will
need to provide a working multirate config in the channel activation,
otherwise the channel act might be nacked.

Copy the config from the current lchan into the new lchan. The
abis code simply added the mrconf if the speech mode was amr.
Before this commit the invalidate mrconf with all zeroes was
copied/sent.