nanokernel: Add back pointer to microkernel task

Adds a back pointer to the microkernel task to the TCS when
configured for a microkernel. This is a necessary prerequisite
to support microkernel tasks pending on nanokernel objects.

Change-Id: Ia62f9cf482ca20b008772dad80cbfd6acb6f5b7a
Signed-off-by: Peter Mitsis <peter.mitsis@windriver.com>