tools/genboardscfg.py: pick up also commented maitainers

We are still keeping invalid email addressed in MAINTAINERS
because they carry information.

The problem is that scripts/get_maintainer.pl adds emails in the
"M:" field including invalid ones.

We want to comment out invalid email addresses in MAINTAINERS
to prevent scripts/get_maintainer.pl from picking them up.
On the other hand, we want to collect them for boards.cfg
to know the last known maintainer of each board.

This commit adjusts tools/genboardscfg.py to parse also
the commented "M:" fields, which is useful for the next commit.

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