twister: automatically add module board roots

Twister does not handle out-of-tree boards automatically. Modules like
e.g. example-application that define their own board root folder, are
required to inform twister via `--board-root`. In contrast, west is able
to to this automatically. This patch makes twister a bit smarter by
automatically filling the board root list with module paths as well.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>