boards/common: cleanup documentation

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boards/common/doc.txt

 /**
-Copyright (C) 2017 HAW Hamburg
-
-This file is subject to the terms and conditions of the GNU Lesser
-General Public License v2.1. See the file LICENSE in the top level
-directory for more details.
-
 @defgroup    boards_common Common Board Groups
 @ingroup     boards
 @brief       Common definitions and implementations for board groups