tools/genboards.cfg: be tolerant of insane Kconfig

tools/genboards.py expects all the Kconfig and defconfig are written
correctly.  Assume some developer accidentally broke his board.
Error-out just for one broken board is annoying for the other
developers.  Let's skip insane boards and continue processing.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
tools:genboards.cfg: wait all other subprocesses before error-out

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

We have to make sure there is no subprocess (in this case,
"make O=<work_dir> ...") using the work directory.
Otherwise the subprocess spits lots of error messages and might
cause more problems.
Perhaps some users get upset to see too much error messages.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
fix comment
fix2