unified: streamline "timeout add" internal interfaces.

Rename _do_timeout_add to _add_timeout, rename _TIMEOUT_ADD to
_add_thread_timeout to better reflect their functionalities. Have the
latter call the former, remove _do_timeout_add and
_nano_timer_timeout_add.

Change-Id: Ica86bea10d99d72bf78379598a942d277e7002d0
Signed-off-by: Benjamin Walsh <benjamin.walsh@windriver.com>
6 files changed