boards/mega-xplained: Initial Mega1284P Xplained support

boards/mega-xplained/Makefile

boards/mega-xplained/Makefile.dep

boards/mega-xplained/Makefile.features

boards/mega-xplained/Makefile.include

boards/mega-xplained/board.c

boards/mega-xplained/dist/debug.sh

boards/mega-xplained/dist/debug_srv.sh

boards/mega-xplained/dist/gdb.conf

boards/mega-xplained/doc.txt

boards/mega-xplained/include/adc_params.h

boards/mega-xplained/include/board.h

boards/mega-xplained/include/gpio_params.h

boards/mega-xplained/include/mega-xplained_pinmap.h

boards/mega-xplained/include/periph_conf.h