unified: Rationalize thread priority APIs

* Gets rid of k_current_priority_get(). Users can just call
  k_thread_priority_get(k_current_get()) instead.

* Declares k_thread_priority_get() in kernel.h, where it
  really belongs.

* Removes duplicate declaration of k_thread_priority_set().

Change-Id: I616ae6f2e06c95ecba3b92324186b3fa29162fd1
Signed-off-by: Allan Stephens <allan.stephens@windriver.com>
6 files changed