Depthai: [BUG] Udev check in config_manager fails if udev rules with bad permissions exist in /etc/udev/rules.d

[botmayank]

Describe the bug
I had STlink related rules lying around in my /etc/udev/rules.d with permissions preventing the grep command (linuxCheckApplyUsbRules() in config_manager.py) from running on them. All other good rules had octal permission 644 while these had 700. This caused python3 depthai_demo.py to fail and complain about udev rules even though I had properly added the OAK-D ones.

To Reproduce
Steps to reproduce the behavior:

1. Add a rule with file permissions 700. sudo chmod 700 49-stlinkv1.rules
2. Try launching the demo app: python3 depthai_demo.py
3. It'll fail with the log for udev rules:

Depthai version installed:  0.4.1.1

WARNING: Usb rules not found

Set rules: 
echo 'SUBSYSTEM=="usb", ATTRS{idVendor}=="03e7", MODE="0666"' | sudo tee /etc/udev/rules.d/80-movidius.rules 
sudo udevadm control --reload-rules && udevadm trigger 
Disconnect/connect usb cable on host! 

Expected behavior
The demo app should've launched.

SOLUTION
Simply changing the octal permissions for the offending rules fixed the issue.

sudo chmod 644 <annoying.rules>

[botmayank]

Described fix in issue description^^

[Luxonis-Brandon]

Awesome, thanks for taking the time to write this up @botmayank so that others will know how to fix this when running into the same or similar issue!