Configuring the network interface with DHCP via
/etc/network/interfaces generally does not work when NFS booting.  The
DHCP configuration will initially bring the interface down and system
then hangs at that point as the root filesystem is no longer
accessable.

This adds a script to be run via a pre-up line in
/etc/network/interfaces.  It checks for a NFS root filesystem and if
the interface to be configured is used for access to the NFS server.
If this is the case, then it returns a failure code so that ifup will
not configure the interface.  This works for DHCP and another config
methods (static, bootp, etc.).  This system does detect if the
interface to be configured isn't the one used for NFS and doesn't skip
it when that is the case.

NFS filesystems that aren't the root fs aren't considered.

Fixes bug #4790.

Signed-off-by: Trent Piepho <tpiepho@kymetacorp.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>