Clean up private APIs that mark context as (non)essential

1) Renames APIs to align them with conventions used by other
   general-context nanokernel APIs.

2) Relocates implementation of these APIs to the architecture-
   independent portion of the nanokernel.

Change-Id: I1aa60029aaa96697cd8fcb594bbae23ba6656661
Signed-off-by: Allan Stephens <allan.stephens@windriver.com>