kernel/sched: optimize sched_yield with CONFIG_SCHED_YIELD_OPTIMIZATION
kernel/sched: optimize sched_yield with CONFIG_SCHED_YIELD_OPTIMIZATION

1. No need to check lockcount
  Because this function yields the CPU resource, no need to check
  lockcount. It's the same as other function like sleep, sem_wait,
  mq_receive and so on. Even the running thread locks, if running
  thread calls context switching, it works. The lockcount just blocks
  context switching from outside.
  Due to this, remove checking lockcount and change sched_addreadytorun
  to sched_addprioritizied.
2. Move the location of calling sched_mergepending and Get new head of list
  Next tcb (rtcb->flink) is getting at start of yield function.
  But after sched_mergepending, the head of list could be changed so that
  old ntcb should not be used to do context switching.
  Final sequence is as below
  1) remove current tcb to yield
  2) merge pending
  3) get the head of the list
  4) do context switching with the tcb which comes at 3rd step
3. Remove checking 'switch_needed'
  The if condition compared current priority to next priority is enough
  to check the necessarity of switching. It does not need to check it
  anymore.
4. Add initialize the timeslice of new tcb
  Because of RR, it should be initialized before starting.

Signed-off-by: sunghan-chang <c954e25285721d8faa8b1a5c1ae2ef5a35338d32@samsung.com>

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