If board `plank` was deprecated, then building with `-DBOARD=plank/foo`
would fail, so as to discourage identifier misuse. However, `plank/foo`
could actually be a valid target in HWMv2, so this was a bad idea.
Replace the offending error with more open-ended identifier handling,
originally suggested in #68940 (anchor: #discussion_r1492103361).
Signed-off-by: Grzegorz Swiderski <grzegorz.swiderski@nordicsemi.no>
083036fffc