scripts: list_boards: Reintroduce Board.dir as @property

In the `Board` class, the `dir` member was renamed to `directories`,
both to indicate that it is now a list (in HWMv2 with board extensions)
and to reflect the addition of the `BOARD_DIRECTORIES` CMake variable.
Considering that the build system also keeps the `BOARD_DIR` variable,
and for the sake of backwards compatibility and brevity, it should be
useful to retain `Board.dir` in Python as well, symmetrically.

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