Remove private context structure from public nanokernel API

The tCCS type doesn't need to be publicly exposed; any public APIs
that need to use this kind of type should be using nano_context_id_t.

Change-Id: Ife1e73c4a21326bf54e2d52bfa1f1281245935a1
Signed-off-by: Allan Stephens <allan.stephens@windriver.com>