kernel: Restore CONFIG_MULTITHREADING=n behavior

The prepare_multithreading()/switch_to_main_thread() steps were being
done unconditionally, when with multhreading disabled we want to jump
straight into the main thread on the existing stack.

Needless to say, that doesn't work well.  Fixes #8361.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>
1 file changed