doxygen: use ingroup instead of addtogroup in core

core/include/assert.h

core/include/bitarithm.h

core/include/byteorder.h

core/include/cib.h

core/include/clist.h

core/include/debug.h

core/include/kernel_defines.h

core/include/kernel_init.h

core/include/kernel_types.h

core/include/lifo.h

core/include/list.h

core/include/log.h

core/include/panic.h

core/include/priority_queue.h

core/include/ringbuffer.h