Correctly write the NWK update ID when forming a network

[puddly]

The SiLabs SDK unfortunately does not actually have a public API to set the network update ID. The ID set during formation is ignored. To work around this, we directly write the update ID into NVRAM if it is not 0.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.44%. Comparing base (0a00d81) to head (83c0def).

Additional details and impacted files

@@           Coverage Diff           @@
##              dev     #679   +/-   ##
=======================================
  Coverage   99.44%   99.44%           
=======================================
  Files          52       52           
  Lines        3769     3789   +20     
=======================================
+ Hits         3748     3768   +20     
  Misses         21       21           

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