kernel: move K_ESSENTIAL from thread_state to execution_flags

The execution_flags will store the user-facing states of a thread.

This also fixes a bug where K_ESSENTIAL was already assigned to
execution_flags via the options field of
k_thread_spawn()/K_THREAD_DEFINE().

Change-Id: I91ad7a62b5d180e09eead8985ff519809959ecf2
Signed-off-by: Benjamin Walsh <walsh.benj@gmail.com>