support a make target for building gadgets

[jawn-smith]

Leaving as a draft as no new tests have been written.

[codecov]

Codecov Report

Merging #117 (0835665) into feature/classic-image-redesign (435d899) will not change coverage.
The diff coverage is 100.00%.

@@                       Coverage Diff                        @@
##           feature/classic-image-redesign      #117   +/-   ##
================================================================
  Coverage                          100.00%   100.00%           
================================================================
  Files                                   9         9           
  Lines                                1913      1915    +2     
================================================================
+ Hits                                 1913      1915    +2     

Impacted Files	Coverage Δ
internal/imagedefinition/image_definition.go	100.00% <ø> (ø)
internal/statemachine/classic_states.go	100.00% <100.00%> (ø)

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[mwhudson]

code looks ok, don't have enough context to know if this behaviour is what users expect :)

[sil2100]

Yeah, it's as we discussed. This feels like a sane design decision. My only proposition would be to maybe use GadgetTarget instead of MakeTarget, but both names are good.