grub: use extra_kernel_args

When playing with my d2500cc, I found that extra_kernel_args was ignored.
So this patch permit to boot with grub and either NFS/ramdisk/NBD with
extra_kernel_args.
16 jobs for grub-extra_kernel_args in 16 minutes and 53 seconds (queued for 3 seconds)
latest
Status Job ID Name Coverage
  Test
passed #119009
amd64
dispatcher-debian-10

00:02:18

passed #119011
amd64
dispatcher-debian-11

00:02:33

passed #119010
amd64
server-debian-10

00:03:00

passed #119012
amd64
server-debian-11

00:05:39

 
  Analyze
passed #119013
amd64-dind
bandit-sast

00:01:01

passed #119014
amd64
black

00:00:35

passed #119015
amd64
code_quality

00:00:24

passed #119020
amd64
codespell

00:00:22

passed #119016
amd64
coverage

00:06:31

60.92%
passed #119017
amd64
dockerfiles

00:00:18

passed #119019
amd64
pylint

00:03:09

passed #119018
amd64
schemas

00:00:32

 
  Build
passed #119022
amd64
debian/10

00:01:58

passed #119021
amd64
doc

00:01:18

passed #119023
amd64-dind
docker-amd64-dispatcher

00:04:13

passed #119024
amd64-dind
docker-amd64-server

00:04:40