Prepare switch from legacy network interfaces to systemd-networkd

[jkirk]

By default grml-debootstrap used the host /etc/network/interfaces file to configure the network of the guest system. We now provide a systemd-networkd configuration enabling DHCP on all network devices next to the legacy /etc/network/interfaces which only enables DHCP on eth0 (which does not work when using the predictable devices names).

If no networking option is given, this will be the new default.

NOTE: systemd-networkd is currently not enabled by default.

[jkirk]

@mika Currently systemd-networkd is not enabled on the target system.

Should we enable it by default (at least for Debian/stretch+ releases)?

[jkirk]

@mika: I updated the PR. Please review. Thx.

[jkirk]

@mika; And yes, the next step is to enable systemd-networkd. But I am not sure if this should be part of this PR.

[mika]

Via @zeha and https://twitter.com/CyrilBrulebois/status/1026935974931116032 ->

udevadm info -e | egrep 'ID_NET_NAME_(ONBOARD|PATH)'

is our friend to identify the device name network devices will get assigned, independent of their current naming, JFTR :)

[mika]

FTR, I'd like to get this integrated rather sooner than later. :)

[paulmenzel]

@jkirk, it would be awesome if you could rebase this.