k_thread_abort(): assert if abort essential thread

Previously, this was only done if an essential thread self-exited,
and was a runtime check that generated a kernel panic.

Now if any thread has k_thread_abort() called on it, and that thread
is essential to the system operation, this check is made. It is now
an assertion.

_NANO_ERR_INVALID_TASK_EXIT checks and printouts removed since this
is now an assertion.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>
15 files changed