CODEOWNERS: Improve missing entry detection

In order to handle missing entries in the CODEOWNERS file properly, a
few changes are introduced in both the CODEOWNERS file and the script
that checks for missing entries (scripts/ci/codeowners.py):

- Add an '*' entry in the CODEOWNERS file. This ensures that GitHub
  assigns someone (in this case ncs-code-owners) by default whenever a
  PR introduces a file that is not covered by an entry in the file
- Remove the top-level section entries (i.e. applications/, samples/,
  subsys/, etc). This is so that the script can catch missing files,
  since catch-all entries like those mask those missing files
- Skip the processing of the '*' top-level entry in the script. This is
  so that, while GitHub assigns someone to the PR, the entry doesn't
  maks out every other missing file in PRs

Also take the opportunity to add missing entries and sort the
sections alphabetically.

Signed-off-by: Carles Cufi <[email protected]>