kernel/Makefile: clean update

kernel/Makefile:
	clean target missed some files.
MUTEX: fix stacktrace

kernel/mutex.s:
	due to mutex_unlock preserving flags, the return address is located
	elsewhere on the stack than is the case with the mutex_(spin)lock.
	Remove the stacktrace calls from mutex_fail$ to the three methods.
Update FILES (directory/file overview)
Describe design flaw in interrupt/mutex/scheduler.

kernel/net/net.s:
	Ever since the strict mutex checking there have been cases where
	net_rx_packet_task would be unable to acquire the NET lock.

	Describe why this happens and what the solution is.