Fix permissions for system user at startup

[James-Livesey]

Further to LiveGTech/OS-Bootstrap#5, we will need to update startup.sh to reflect the changes for that issue. This requires a replacement of sudo chmod -R a+w /system with sudo chmod -R ug+w /system.

We can revert the current incorrect permissions on the /system folder by running sudo chmod -R o-w /system, which removes the write permission for other users for all files and directories.