boards: arm: twr_ke18f: only enable HW stack protection if !userspace

Only enable hardware stack protection by default on the NXP TWR-K18F
development board if userspace is not enabled.

The NXP KE1xF SoC has 8 MPU regions, which is insufficient for using HW
stack protection and userspace simultaneously.

Fixes bc9a498b.

Signed-off-by: Henrik Brix Andersen <hebad@vestas.com>