tools/genboardscfg.py: ignore defconfigs starting with a dot

Kconfig in U-Boot creates a temporary file configs/.tmp_defconfig
during processing "make <board>_defconfig".  The temporary file
might be left over for some reasons.

Just in case, tools/genboardscfg.py should make sure to
not read such garbage files.

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