cmake: clear BOARD_CACHE when invalid board identifier is given

Clear BOARD_CACHE when no or an invalid board identifier is provided
on first CMake invocation.

This allows users to re-run CMake and provide a valid board identifier
as well as avoiding `BOARD` to be replaced with an invalid BOARD_CACHED
value.

Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>

cmake/modules/boards.cmake

@@ -255,6 +255,7 @@ elseif(HWMv2)
 
     if(NOT ("${BOARD}${BOARD_IDENTIFIER}" IN_LIST BOARD_IDENTIFIERS))
       string(REPLACE ";" "\n" BOARD_IDENTIFIERS "${BOARD_IDENTIFIERS}")
+      unset(CACHED_BOARD CACHE)
       message(FATAL_ERROR "Board identifier `${BOARD_IDENTIFIER}` for board \
             `${BOARD}` not found. Please specify a valid board.\n"
             "Valid board identifiers for ${BOARD_NAME} are:\n${BOARD_IDENTIFIERS}\n")