Add support for keeping track of cumulative CPU time for each thread.

miosix/Makefile

miosix/_doc/doxygen/Doxyfile

miosix/config/miosix_settings.h

miosix/kernel/cpu_time_counter.cpp

miosix/kernel/cpu_time_counter.h

miosix/kernel/cpu_time_counter_private.h

miosix/kernel/cpu_time_counter_types.h

miosix/kernel/kernel.h

miosix/kernel/scheduler/scheduler.h

miosix/miosix.h