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.
The new SDK requires 802.15.4 to be enabled in the board DTS file.
Also disable IPv6 explicitly to avoid compile errors related to its
network stack, since it is not used currently.
Closes#94
