config: also save next-server in the state file

The early boot tooling gets the root-path from our state file due to a
better way to do that. However, when booting with NFS root, the root path
alone is not sufficient; the server address is communicated via the
next-server option. Save that one in the state file as well.
12 jobs for lr/next-server in 37 minutes and 6 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Test
passed #321671
allowed to fail
checkpatch

00:01:35

passed #321678
t_debian:9

00:23:35

passed #321680
t_debian:sid

00:23:43

manual #321679
allowed to fail manual
t_debian:testing
passed #321672
t_fedora:28

00:32:13

passed #321673
t_fedora:29

00:36:21

passed #321674
t_fedora:30

00:29:08

manual #321675
allowed to fail manual
t_fedora:rawhide
passed #321676
t_ubuntu:16.04

00:24:08

passed #321677
t_ubuntu:18.04

00:24:08

 
  External
failed NetworkManager test verification

failed NetworkManager test verification

06:11:55