kernel: add _IS_IDLE_TASK()

Add a way of finding if a microkernel task is the idle task instead of
of duplicating:

  task_priority_get() == (CONFIG_NUM_TASK_PRIORITIES - 1)

which could be subject to change.

Only available for microkernel, since there is no such concept in the
nanokernel.

Change-Id: Ie8930981f1a2ac5ff16f905f4eb4e333c8b59c5d
Signed-off-by: Benjamin Walsh <benjamin.walsh@windriver.com>