tools/genboardscfg.py: be tolerant of insane Kconfig

The tools/genboardscfg.py expects all the Kconfig and defconfig are
written correctly.  Imagine someone accidentally has broken a board.
Error-out just for one broken board is annoying for the other
developers.  Let the tool skip insane boards and continue processing.

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