tools/genboardscfg.py: fix a bug of MAINTAINERS handling

This patch fixes a minor problem:
If a block without "F:   configs/*_defconfig" is followed by another
block with "F:   configs/*_defconfig", the maintainers from the
former block are squashed into the latter.

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