Add Thread::PKgetPriority() to improve API consistency

miosix/kernel/kernel.cpp

miosix/kernel/kernel.h

miosix/kernel/scheduler/control/control_scheduler.cpp

miosix/kernel/scheduler/control/control_scheduler.h

miosix/kernel/scheduler/edf/edf_scheduler.h

miosix/kernel/scheduler/priority/priority_scheduler.cpp

miosix/kernel/scheduler/priority/priority_scheduler.h

miosix/kernel/scheduler/scheduler.h

miosix/kernel/sync.cpp