kernel: Init back pointer to microkernel task

Fibers initialize this back pointer to NULL as they are (by definition)
not microkernel tasks.  Microkernel tasks initialize it to their
corresponding 'ktask_t'.

However for nanokernel systems, the back pointer is always NULL. This
is because there is only one task in a nanokernel system (the background
task) and it can not pend on a nanokernel object--it must poll.

Change-Id: I9840fecc44224bef63d09d587d703720cf33ad57
Signed-off-by: Peter Mitsis <peter.mitsis@windriver.com>