OpenWrt failsafe leaks non-modified packet during boot

[Bloup1net]

Found while working on IPv6 detection, OpenWrt failsafe mode sends IPv4 and IPv6 packets as described in the documentation: https://openwrt.org/docs/guide-user/troubleshooting/failsafe_and_factory_reset. This can get phantap detected. There is no easy solution to disable it at runtime, as this configuration is in U-Boot. The U-Boot partition is mounted as read-only and it's configuration can only be accessed and modified from the U-Boot shell (via UART on the GL-AR150).
The easier solution is to compile OpenWrt with the TARGET_PREINIT_DISABLE_FAILSAFE option.

Todo: Update Readme and add warning at phantap start.