VLAN support via boot option "vlan=<vid>:<phydevice>"

This provides support for something like:

  ip=10.10.10.42::10.10.10.1:255.255.255.0:grml:eth0:off vlan=301:eth0

to use VID 301 for device eth0.

The implementation tries to be close to what dracut provides with
its vlan boot option implementation, see
http://thread.gmane.org/gmane.linux.kernel.initramfs/2685
Though we aren't 100% identical because we only use <vid>
(e.g.: 3) instead of dract's <vlanname> (e.g.: VLAN_PLUS_VID
(vlan0003), VLAN_PLUS_VID_NO_PAD (vlan3), DEV_PLUS_VID (eth0.0003),
DEV_PLUS_VID_NO_PAD (eth0.3)) options but that might be an option
once we can bring vlan support into initramfs-tools itself.

The vlan-raw-device configuration option in
/etc/network/interfaces isn't really needed technically, but we
want to make the presence and usage of VLANs more obvious for
its users.

Development sponsored by Sipwise Gmbh