boards/*: cleanup arduino-due and udoo doc

- display brief description instead of copyright
- add a reference to the 'real' documentation, otherwise it's empty

boards/arduino-due/doc.txt

 /**
-Copyright (C) 2017 Freie Universität Berlin
-
-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_arduino-due Arduino Due
 @ingroup     boards
 @brief       Support for the Arduino Due board
+
+@see boards_common_arduino_due for details
  */

boards/udoo/doc.txt

 /**
-Copyright (C) 2017 Freie Universität Berlin
-
-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_udoo UDOO
 @ingroup     boards
 @brief       Support for the UDOO board
+
+@see boards_common_arduino_due for details
  */