Add NBD support to grub

Boards booting with grub cannot currently use NBD for storing their
rootfs.
This patch adds support for NBD when booting with grub method.
15 jobs from grub_nbd in 10 minutes and 8 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Test
passed #33112
amd64
dispatcher-debian-10

00:02:39

passed #33110
amd64
dispatcher-debian-9

00:02:36

passed #33113
amd64
server-debian-10

00:01:54

passed #33111
amd64
server-debian-9

00:02:21

 
  Analyze
passed #33114
amd64
black

00:00:28

passed #33116
amd64
code_quality

00:00:23

passed #33115
amd64
codestyle

00:00:37

passed #33119
amd64
job-schema

00:00:39

passed #33117
amd64
pylint

00:02:16

passed #33118
dind amd64 allowed to fail
sast

00:03:24

 
  Build
passed #33122
amd64
amd64/pkg-debian-10

00:01:36

passed #33121
amd64
amd64/pkg-debian-9

00:01:51

passed #33120
amd64
doc

00:01:18

 
  Docker
passed #33123
dind amd64
amd64/lava-dispatcher

00:00:44

passed #33124
dind amd64
amd64/lava-server

00:02:10