libftdispi modified for higher throughput

Based on https://storage.googleapis.com/google-code-archive-downloads/v2/code.google.com/libftdispi/libftdispi-0.2.3.tar.bz2

Modified to use:

• libftdi1
• ftdi1 asynchronous read feature
• "SEND_IMMEDIATE" to avoid FTDI latency timer

Observed shortcomings:

• SPI data send from target during write to target is not captured