add 'struct timer_list' timer code similar to linux + OpenBSC

add 'struct timer_list' timer code similar to linux + OpenBSC

We now support arbitrary timers by means of 'struct timer_list'.
Any part of the program can register such a callback by means
of schedule_timer() on a millisecond-granularity.  However, there
is no guarantee on the timer precision.  It will not execute before
the timer expires - but it might expire quite a bit later than we
have asked it for, depending on how busy the cpu is with other work.

The timer code is in the 'comm/' directory, as it is intended to be
migrated into libosmocore soon.

Furthermore, as we currently don't yet have a scheduler or tasks,
the main() routine explicitly has to call update_timers() to check
for any expired timers and run them.