discover/grub: Allow to set a default index for BLS entries
When the BLS support was added, the conclusion was that default indexes
didn't apply for BLS snippets. But for GRUB 2 the indexes refers to the
boot menu entries in memory, regardless of how these were generated.
Since in GRUB 2 is valid to set a default index even for menu entries
generated from BLS fragments, allow this to also be done in Petitboot.
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com> Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>