This change adds a compile time option to make TFTP the default network protocol.
While TFTP may be selected by specifying it in the URI scheme for root-path, this requires a BOOTP/DHCP daemon to be running to supply this information.
With QEMU, it is possible to specify a directory to simulate a TFTP server and also specify a bootfile to be fetched from this TFTP server (-netdev user,tftp=<dir>,bootfile=<path>).
Unfortunately, there is no option to supply a root-path, which makes it impossible to netboot using QEMU only, without additional daemons.