tools/genboardscfg.py: wait for unfinished subprocesses before error-out

When an error occurs or the program is terminated by the user
on the way, the destructer __del__ of class Slot is invoked and
the work directories are removed.

We have to make sure there are no subprocesses (in this case,
"make O=<work_dir> ...") using the work directories before
removing them.  Otherwise the subprocess spits a bunch of error
messages possibly causing more problems.  Perhaps some users
may get upset to see too many error messages.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: Simon Glass <sjg@chromium.org>