scripts: remove board name from the qualifiers in list_boards.py

Fixes: #69329

The board name was printed as part of printing board qualifiers because
those was being concatenated in the `board_v2_qualifiers()` method.

Keep the qualifiers separated from the board name and let the caller
concatenate the strings when required.

Completion scripts are also updated to handle the corrected behaviour.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>
(cherry picked from commit 66b475a3)