discover/grub2: Allow to separate the --id argument using a space char

[petitboot] / test / parser / data / native-short.conf

name Ubuntu
image /boot/vmlinux-4.15.0-22-generic
initrd /boot/initrd.img-4.15.0-22-generic
args root=UUID=09d1034f-3cff-413a-af22-68be1fa5e3d8 ro