Conditionally load EDID on device with TC358743 capture chip (#1730)

Resolves #1726

This PR only starts the `load-tc358743-edid.service` when a TC358743
capture chip is installed. This change avoids reporting the service as
"failed" when a TC358743 capture chip is not installed on the device.

**Caveat:** The `load-tc358743-edid.service` process is still marked as
`(code=exited, status=1/FAILURE)`, but it's active state is no longer
`failed (Result: exit-code)`. Instead, its active state is `inactive
(dead) (Result: exec-condition)`.

Example output of service being skipped:
```
Jan 28 18:41:02 raspberrypi systemd[1]: Starting Load EDID for TC358743 HDMI capture chip....
Jan 28 18:41:02 raspberrypi systemd[1]: load-tc358743-edid.service: Control process exited, code=exited, status=1/FAILURE
Jan 28 18:41:02 raspberrypi systemd[1]: load-tc358743-edid.service: Skipped due to 'exec-condition'.
Jan 28 18:41:02 raspberrypi systemd[1]: Condition check resulted in Load EDID for TC358743 HDMI capture chip. being skipped.
```

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debian-pkg/debian/tinypilot.load-tc358743-edid.service

@@ -8,6 +8,10 @@ StartLimitBurst=20
 [Service]
 Type=oneshot
 User=root
+ExecCondition=grep \
+  --regexp '^dtoverlay=tc358743$' \
+  --silent \
+  /boot/config.txt
 ExecStart=v4l2-ctl \
   --set-edid=file=/home/ustreamer/edids/tc358743-edid.hex \
   --fix-edid-checksums