west: blobs: Ensure that folder exists before fetching

The `<module>/zephyr/blobs/` folder, as well as any additional
sub-folders that might be required under `blobs/` may not exist when
fetching a blob. Ensure that the folder exists, create it if it doesn't.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>