buildman: Fix problem with non-existent output directories

Now that we have buildman telling genboards.cfg to use an output
directory we need to ensure that it exists.

Cc: Bin Meng <bmeng.cn@gmail.com>
Cc: Simon Glass <sjg@chromium.org>
Fixes: bc750bca ("tools: buildman: Honor output directory when generating boards.cfg")
Signed-off-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>