The timing advance controller that is implemented in loops.c of
osmo-bts-trx only works for osmo-bts-trx and not for any of the phy
based bts. Lets move the timing advance controller into the common part
and make it available for every bts. Also lets add a unit-test.
Change-Id: If7ddf74db3abc9b9872abe620a0aeebe3327e70a
Related: SYS#4567