--loopback option does not allow you to specify tile

[ed-xmos]

Currently --loopback arguments apply to all tiles. This should be more specific so different loopbacks can be done on different tiles.
Something like:

--loopback tile[n] XS1_PORT_XX XS1_PORT_XX

[ed-xmos]

For reference, xsim uses this format:

--plugin LoopbackPort.dll '-port tile[0] XS1_PORT_16A 16 0 -port tile[0] XS1_PORT_16B 16 0' | grep -