microkernel: Add support for *_sleep() during initialization

Add support for task_sleep() and fiber_sleep() during the
system initialization. When CONFIG_NANO_TIMEOUTS defined,
before the k_server() starts, kernel uses nanokernel
system clock announce and task sleep functionality.

To give device drivers early sleep functionality, the system
clock has to start on SECONDARY initialization level, same
as most of the drivers.

Change-Id: Ie1d391945cd1cfb9a5dc199783c2d224eb1b0ef3
Signed-off-by: Dmitriy Korovkin <dmitriy.korovkin@windriver.com>