Support device img files

[ernoaapa]

Updated build device to produce img -file as default output which is easy to write into disk with dd.

[codecov[bot]]

Codecov Report

Merging #25 into master will decrease coverage by 0.4%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #25      +/-   ##
==========================================
- Coverage   26.02%   25.61%   -0.41%     
==========================================
  Files          50       50              
  Lines        2425     2420       -5     
==========================================
- Hits          631      620      -11     
- Misses       1724     1731       +7     
+ Partials       70       69       -1

Impacted Files	Coverage Δ
pkg/cmd/build/utils.go	53.19% <100%> (ø)	:arrow_up:
pkg/runtime/utils.go	0% <0%> (-84.62%)	:arrow_down:
pkg/runtime/containerd.go	1.72% <0%> (ø)	:arrow_up:

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