Merge GPIO and flow control bootloader reset

[puddly]

To allow other reset patterns to be used in the future (if needed) and to consolidate the GPIO logic, I've merged the GPIO and flow control bootloader reset patterns. This is in preparation for flow control in the web flasher: NabuCasa/sl-web-tools#20

[puddly]

Should fix darkxst/silabs-firmware-builder#172 with the above patch, @darkxst

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 32.81250% with 43 lines in your changes missing coverage. Please review.

Project coverage is 66.23%. Comparing base (b009196) to head (ad6d6f5).
Report is 14 commits behind head on dev.

Files with missing lines	Patch %	Lines
universal_silabs_flasher/gpio.py	25.00%	15 Missing ⚠️
universal_silabs_flasher/common.py	20.00%	12 Missing ⚠️
universal_silabs_flasher/flasher.py	7.69%	12 Missing ⚠️
universal_silabs_flasher/flash.py	0.00%	4 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##              dev      #94      +/-   ##
==========================================
+ Coverage   61.78%   66.23%   +4.44%     
==========================================
  Files          14       15       +1     
  Lines        1748     1851     +103     
==========================================
+ Hits         1080     1226     +146     
+ Misses        668      625      -43     

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