tools/genboardscfg.py: fix minor problems on termination

This tool deletes the incomplete boards.cfg
if it encounters an error or is is terminated by the user.

I notice some problems even though they rarely happen.

[1] The boards.cfg is removed if the program is terminated
during __gen_boards_cfg() function but before boards.cfg
is actually touched.  In this case, the previous boards.cfg
should be kept as it is.

[2] If an error occurs while deleting the incomplete boards.cfg,
the program throws another exception.  This hides the privious
exception and we will not be able to know the real cause.

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