add comma serial to tools

old-commit-hash: 29499463375c659e8c830bf06996a9964c7aaf79

tools/serial/README.md

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+# comma serial
+
+The comma serial gets you access to a low level serial console on your comma three, while providing astable 12V to power the device.
+
+The serial is available on the [comma shop](https://comma.ai/shop/products/comma-serial).
+
+## setup
+
+* Connect all three cables to the serial
+* Connect the USB A to your computer
+* Connect the USB-C to the OBD-C port on your comma three
+
+## usage
+
+```
+sudo screen /dev/ttyUSB0 115200
+```
+or use `connect.sh` to run the previous command in a loop

tools/serial/connect.sh

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+#!/bin/bash
+
+while true; do
+  if ls /dev/ttyUSB* 2> /dev/null; then
+    sudo screen /dev/ttyUSB* 115200
+  fi
+  sleep 0.1
+done