cmake: boards: Error out if multiple boards use the same name

Detecting this requires `boards.cmake` being able to handle multi-line
output from `list_boards.py`. Implement a similar line reading loop to
the one used in `hwm_v2.cmake`.

Failing to handle this could result in an incorrectly parsed list of
valid board qualifiers. Here's the expected list for `nrf52_bsim`:

  - "native"

and here's what would happen if two copies of that board were found:

  - "native\nNAME"
  - "nrf52_bsim"
  - "native"

Instead of that, there would now be a proper error message listing all
board directories which contain `nrf52_bsim`.

Signed-off-by: Grzegorz Swiderski <grzegorz.swiderski@nordicsemi.no>