Refactor elemental

[davidcassany]

This commit refactors elemental image deploy procedure to always create and prepare the root tree in a temporary folder before synching it to a filesystem image most likely as a mounted loop device.

This change has a couple of immediate benefits:

1. We can pre-compute the root tree size before creating the filesystem image. Hence image sizes can be adjusted to root-tree size. Currently in this PR the default size of the images is set to 0 which means it will use as much as the image requires plus and arbitrary overhead of 256MB (we could consider making it configurable if needed). The use of an static size is still possible by setting the partition size to any value greater than 0.

2. There is no path differentiation between filesystems. The root tree is prepared following the same logic independently of the target filesystem. Squashfs is no longer an exception, building the image follows the same logic as it was any other writable filesystem.

We also can argue this is a simple, robust and flexible logic compared with the previous code.

The counter part is having to prepare the root-tree to later on copy it to the final image, this causes the root-tree to be written twice, one to prepare it and another one to sync it to the target location.

Fixes #172

[kkaempf]

I'd recommend to put the copyright year changes in a separate PR

[codecov]

Codecov Report

Base: 75.94% // Head: 75.76% // Decreases project coverage by -0.17% ⚠️

Coverage data is based on head (142932c) compared to base (be54c07).
Patch coverage: 82.71% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #404      +/-   ##
==========================================
- Coverage   75.94%   75.76%   -0.18%     
==========================================
  Files          61       61              
  Lines        6189     6284      +95     
==========================================
+ Hits         4700     4761      +61     
- Misses       1160     1187      +27     
- Partials      329      336       +7     

Impacted Files	Coverage Δ
pkg/constants/constants.go	95.31% <ø> (ø)
pkg/config/config.go	89.70% <33.33%> (-0.72%)	⬇️
pkg/utils/fs.go	59.62% <63.63%> (-1.71%)	⬇️
pkg/action/upgrade.go	61.80% <69.23%> (+0.33%)	⬆️
pkg/utils/common.go	80.34% <77.77%> (-0.16%)	⬇️
pkg/action/reset.go	68.02% <84.00%> (+1.74%)	⬆️
pkg/elemental/elemental.go	85.97% <84.15%> (-2.24%)	⬇️
pkg/action/install.go	80.64% <91.17%> (-0.95%)	⬇️
pkg/http/client.go	85.29% <0.00%> (-14.71%)	⬇️
... and 7 more

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[kkaempf]

lgtm 🤞🏻

[frelon]

Works like a charm!