kernel: Make idle task sleep

Device drivers may use task_sleep() routine during the initialization.
As device driver initialization is carried by idle task, it can not
be sheduled out as any other task. Idle task goes into a wait loop
instead.

In order to invoke task_sleep() device drivers must enable
CONFIG_NANO_TIMEOUTS option.

Change-Id: Ib73a2ad1f3c0bda44c24f2417e102bfaa3a13a15
Signed-off-by: Dmitriy Korovkin <dmitriy.korovkin@windriver.com>