Display a warning for unsupported boards
Display a warning for unsupported boards

Display a warning rather than an error when the target board is
unsupported. This is not a fatal error, since pyOCD can still connect
and be used to debug. It just cannot flash the target.

matthewelse/pyOCD,mesheven/pyOCD,matthewelse/pyOCD,pyocd/pyOCD,wjzhang/pyOCD,flit/pyOCD,mbedmicro/pyOCD,mesheven/pyOCD,0xc0170/pyOCD,flit/pyOCD,0xc0170/pyOCD,mesheven/pyOCD,mbedmicro/pyOCD,mbedmicro/pyOCD,0xc0170/pyOCD,wjzhang/pyOCD,wjzhang/pyOCD,pyocd/pyOCD,matthewelse/pyOCD