kernel: Treat aborting by main() as a fatal system error

An application-supplied main() routine is now considered to be
essential to system operation. Thus, if main() experiences an
error that aborts the main thread a fatal system error is raised.

Note: If main() completes its work and does a standard return-
to-caller the main thread terminates normally.

Change-Id: Icc9499f13578299244a856a246ad2a7d34a72f54
Signed-off-by: Allan Stephens <allan.stephens@windriver.com>