arm-teensy3 - serial receive hardware flow control

[quozl]

When enabled in build, Teensy 3.1 pin 6 is RTS and must be tied to CTS of host. Host must be configured to observe CTS.

Tested with screen /dev/ttyUSB0 115200,crtscts no issues.

Tested with cat /dev/ttyUSB0 and

for x in $(seq 10000); do
     echo -e "get-msecs .d ( $x )\n" > /dev/ttyUSB0;
done

with no issues.

@jordanhubbard, for your interest, this fixes pasting over UART, at the expense of one more wire.