Rename firmware types and support variant metadata

[puddly]

Implements NabuCasa/silabs-firmware-builder#82

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 62.50000% with 9 lines in your changes missing coverage. Please review.

Project coverage is 61.86%. Comparing base (b009196) to head (2674282).
Report is 5 commits behind head on dev.

Files with missing lines	Patch %	Lines
universal_silabs_flasher/flash.py	0.00%	5 Missing ⚠️
universal_silabs_flasher/firmware.py	66.66%	4 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##              dev      #81      +/-   ##
==========================================
+ Coverage   61.78%   61.86%   +0.08%     
==========================================
  Files          14       14              
  Lines        1748     1757       +9     
==========================================
+ Hits         1080     1087       +7     
- Misses        668      670       +2     

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