In the effort to increse reliability of the Zigbee sample, #126 and #130
introduced handling for possibly rare, hard-to-debug events that may be
occurring. While these changes seem to help in practive, we don't know
exactly how often, if ever, they are triggered.
This PR introduces debug_counters in hope to add some visilibty in these
hard-to-debug, rare conditions.
Calling `prst_debug_counters_increment("some_counter")` will increment a
value stored in flash. These values are then dumped via logging when the
device boots. Moving forward we can also consider exposing these values
via a ZigBee cluster.
