OpenOCD: Make incompatible with SMP

In case of SMP (i.e. multiple execution units processing
the same list of tasks) we cannot use the same data structures for
getting data about active tasks as with just one processor (UP).

So until explicit support of SMP is added make sure we don't allow
to select both OPENOCD & SMP simultaneously.

Moreover starting from
commit a203d219 ("kernel: remove legacy fields in _kernel")
this will lead to build-time error if MP_NUM_CPUS > 1.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>