Getting message "Unknown id: netcfg" while setting up usb0 interface for reverse tethering
There's a slight syntax error in your statement: the first non-option parameter to su (which stands for "Switch User") is the user to switch to (if omitted, root is assumed). So your su netcfg usb0 dhcp basically advises su to "switch to the user netcfg – but there is no such user.

To fix it up, just use su -c "netcfg usb0 dhcp" – which would, due to the non-existent "first non-option argument", would advise su to switch to root and then, by the -c parameter, execute the Command netcfg usb0 dhcp. Further note the command is quoted: this is required as it contains (white) spaces.