lint-stable: Check that modified boards contain meaningful board_info.txt.

Change-Id: Idd3ff029e16b4b963f13d341dabdc1949c4e9275
Signed-off-by: Vladimir Serbinenko <phcoder@gmail.com>
Reviewed-on: http://review.coreboot.org/6670
Tested-by: build bot (Jenkins)
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>