nfs_utils: Fix for read-only rootfs

In order to work on a read-only rootfs, nfs_utils would like to write
stuff to /var/lib/nfs, since it's not a tmpfs, it won't work.

Instead of doing little dances around the filesystem, tell nfs_utils to
use /run/nfs for everyone.

Modify the startup script accordingly.

Signed-off-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>