discover/grub2: Allow to separate the --id argument using a space char
The GRUB menuentry command allows to separate the arguments for options
using either a '=' or a ' '. The latter is the convention used when the
menu entries are defined in the GRUB config file, but this is currently
not supported by Petitboot.
Add tests to cover both using '--id=foo' and '--id foo' as options.
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com> Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>