boards/frdm-kw41z: cleanup documentation

- Cleanup brief description to be consistent with other boards
- Move general documentation in doc.txt

boards/frdm-kw41z/doc.txt

+/**
+ * @defgroup    boards_frdm-kw41z Freescale FRDM-KW41Z Board
+ * @ingroup     boards
+ * @brief       Support for the FRDM-KW41Z
+ */
\ No newline at end of file

boards/frdm-kw41z/include/board.h

@@ -7,9 +7,7 @@
  */
 
 /**
- * @defgroup    boards_frdm-kw41z Freescale FRDM-KW41Z Board
- * @ingroup     boards
- * @brief       Board specific implementations for the FRDM-KW41Z
+ * @ingroup     boards_frdm-kw41z
  * @{
  *
  * @file