Support debian_networking boot option to use Debian's network behavior

Support debian_networking boot option to use Debian's network behavior

Debian's ip=... support diverges from kernel one and we (Grml) prefer
to use the behavior that's used by the kernel. By default we use our
Grml_Networking implementation from components/9990-grml-networking.sh,
but if the debian_networking boot option is used then we use Debian's
live-boot behavior with Netbase from components/9990-netbase.sh.
(Some minor details also diverge in the network handling, with
vlan support present in Grml, different nameserver handling,...)