The macros CONCAT(), MIN(), and MAX() are defined over and over again in
RIOT's code base. This de-duplicates the code by moving the macros to a
common place.
Replace direct accesses to sched_active_thread and sched_active_pid with
the helper functions thread_getpid() and thread_get_active(). This serves
two purposes:
1. It makes accidental writes to those variable from outside core less likely.
2. Casting off the volatile qualifier is now well contained to those two
functions
