The `start_network.sh` script creates a `${TUN}` interface, so
the `${TAP}` variable will always be empty.
This means start_dhcpd() will always fail as the DHCPv6 script is lacking
an interface.
Previously, when the creation of the TUN/TAP interface failed in one of
the `start_network.sh` scripts, the script will fail with a cryptic
error like
> dist/tools/ethos/start_network.sh: 68: [: -eq: unexpected operator
This fix ensures, that the value of this variable checked is always set
such that in the error case, `ethos`/`sliptty` won't start.
