build.py `--dev` flag does not do the same as codal.json `"dev": true`

Looking at build.py, the dev flag is described as "enable developer mode (does not use target-locked.json)":

There is also another way to "enable developer mode" by setting the "target.dev": true key in codal.json. This mode is processed in the CMake files and it read target-locked.json instead of target.json from codal-microbit-v2: https://github.com/lancaster-university/microbit-v2-samples/blob/1a3d47796b2869b2c473da5ae316450d2bedb29e/CMakeLists.txt#L67-L73

The build.py --dev flag it's only used:

When the --update flag is used to update (git pull) the libraries:
- build.py#L61
- In this case a build.py does not invoke any CMake files, so it never uses target-locked.json or target.json
- So in this case, the build.py description of the --dev flag does not quite apply
When the "target" command argument is used (e.g. python build.py codal-microbit --dev)
- build.py#L116
- In this case it completely overwrites the codal.json to include this flag, and incidentally loses all other configured options from the file
- Also, this needs an existing target to be specified in utils/targets.json, which right now does not contain an entry for codal-microbit-v2
- So this is either an obsolete option, or something that needs to be updated for micro:bit V2

With the normal python build.py invocation with the --dev flag does not do anything at all, which is also a bit confusing.

lancaster-university / microbit-v2-samples