gadget: emmc syntax support

[Meulengracht]

This is the first PR of 3 (likely) in the road to emmc support.

The first PR (this one)

• Implement the neccessary syntax support for gadgets to specifiy eMMC hardware boot partitions and which images go there during gadget updates

Next PRs:

• Add support for volume-assignments (gadget,tests: add support for volume-assignments in gadget #14563)
• Add support for eMMC partitions during gadget updates

REF: SNAPDENG-32142

[codecov]

Codecov Report

Attention: Patch coverage is 96.47059% with 3 lines in your changes missing coverage. Please review.

Project coverage is 78.87%. Comparing base (ac897ee) to head (bed97d5).
Report is 52 commits behind head on master.

Files with missing lines	Patch %	Lines
gadget/gadget.go	95.77%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #14533      +/-   ##
==========================================
+ Coverage   78.85%   78.87%   +0.02%     
==========================================
  Files        1079     1083       +4     
  Lines      145615   146157     +542     
==========================================
+ Hits       114828   115285     +457     
- Misses      23601    23675      +74     
- Partials     7186     7197      +11     

Flag	Coverage Δ
unittests	78.87% <96.47%> (+0.02%)	⬆️

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