configure gadget from BELABOOT

[giuliomoro]

Since 355a2c9 there are two virtual network devices enabled over USB: one that should work natively for Windows and one that should work natively for Mac (and both on Linux). These interfaces usb0 and usb1 have the same IP address (192.168.7.2) and this seems to be working just fine in many cases. In a number of situations (e.g.: Linux, Mac with HORNDIS etc), they will both show up and this sometimes is source of confusion for the OS, which cannot resolve 192.168.7.2, or bela.local or both, or it works just fine. E.g.: on macOS 10.11.6+HORNDIS this is pretty much hit and miss (though removing HORNDIS fixes it, and having distinct IP addresses for the two drivers also fixes it)).

Is it worth adding a config file in BELABOOT where you can enable/disable gadgets? This would be sourced by /opt/Bela/bela_gadget.sh and would allow users that fail to log into the board to revert to the method that works for them without having to be able to read/write an ext4 partition.