[alternative] Support multiple west config files per config level

[thorsten-klein]

This is (another) alternative to PR #849. It is easier to discuss this solution while there is already some implementation existing.

Why?

In a big company there are often shared configs for build system and tooling (e.g., west and its configs).
Those generic configs must be applied first and contain some default or fallback values, if the user does not specify any value in any user config.
Depending on the company and team size, multiple different configs need to be stacked.
Remember, that users must always be able to overwrite values from those generic config files within their config.

Proposed Solution

Multiple config files can be specified in the according environment variable (WEST_CONFIG_SYSTEM, WEST_CONFIG_GLOBAL, WEST_CONFIG_LOCAL) for each config level (separated by ':').
The config files are applied in the same order as they are specified, whereby values from later files override earlier ones.

I personally like configuration through env variables, but I fear you don't.
Nevertheless I have prepared this proposal. Maybe it convinces you.

Note: Maybe this mechanism does not make sense for WEST_CONFIG_LOCAL.
This could also be combined with the approach from #849, so that multiple configs per config level are supported with a dropin config directory each. This would be the ultimative solution.

[marc-hb]

This is (another) alternative to PR #849.

I don't see how this is better than #849 dropins, sorry. It seems to have the same complexity (e.g.: how do you append to an existing value?) but with a less convenient and less standard user interface.

[thorsten-klein]

Perfectly fine for me. I just wanted to show it to you for completeness.
I will close this PR now 👍🏻

[marc-hb]

You could have waited for other opinions! They could still come but it's bit less likely in a closed PR.

[codecov]

Codecov Report

❌ Patch coverage is 94.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.48%. Comparing base (a3af732) to head (3c9c122).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/west/configuration.py	92.50%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #867      +/-   ##
==========================================
- Coverage   84.52%   84.48%   -0.04%     
==========================================
  Files          11       11              
  Lines        3366     3396      +30     
==========================================
+ Hits         2845     2869      +24     
- Misses        521      527       +6     

Files with missing lines	Coverage Δ
src/west/app/config.py	95.04% <100.00%> (+1.43%)	⬆️
src/west/configuration.py	82.31% <92.50%> (-2.61%)	⬇️

... and 2 files with indirect coverage changes

[pdgendt]

Honestly, I like the simplicity here. A user that sets the environment variable, to change a config file location, already needs to know what they are doing.

Allowing to pass in multiple files is in line with the ConfigParser's functionality.