Suggestion: Safer codal.json for users & create codal.dev.json for development

[microbit-carlos]

The current codal.json:

• Has the "dev" option enabled, that clones the main branch from all libraries, which is a bit risky for the average user. The locked target commits should be safer.
  • Also, changing the value to "false" doesn't actually work, so it can be confusing as well
  • codal.json target.dev property looks like a boolean but it isn't #63
• Enables debug messages, which are good for debugging, but odd for normal users, who might not expect to see additional data being sent to serial and might not know how to turn it off (it's not really documented anywhere)

This settings are useful for CODAL development, so I suggest to keep them in a new separate codal.dev.json (like codal.ble.json), so that devs can simply mv codal.dev.json codal.json after cloning.

This way we leave a safer codal.json configuration for normal users that might just want to create a CODAL C++ micro:bit programme.

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This PR also remove the test_ignore key because it's used by build.py only when found in the utils/targets.json file. It probably was accidentally added from copy/pasting the config from utils/targets.json.

Also enables partial flashing on codal.ble.json, which we should probably have on by default.

[JohnVidler]

Suggest we also document this in the README - can this be added to this PR as well?

[microbit-carlos]

README updated, this is PR ready for merge, thanks everyone!