configurable mpsse for jtag

[cz172638]

there are several ftdi based jtag probes, like flyswatter2, bus blaster, xds100 and others even all use same/similar chip, mpsse settings are different. in pyftdi it is hardcoded. for example openocd has ftdi_layout_init config option for intial state and direction different for almost each ftdi adapter. and ftdi_layout_signal for custom signals

[eblot]

I would accept patches, but given than PyFtdi JTAG support is very basic, I'm not sure it is even worth the workload.