tools/genboardscfg.py: be tolerant of missing MAINTAINERS

tools/genboardscfg.py expects all the boards have MAINTAINERS.
If someone adds a new board but misses to add its MAINTAINERS file,
tools/genboardscfg.py fails to generate the boards.cfg file.
It is annoying for the other developers.

This commit allows tools/genboardscfg.py to display warning messages
and continue processing even if some MAINTAINERS files are missing
or have broken formats.

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