Previously, grub_scan_disk could set ata, ahci or "both",
which would make both be tried (ahci first). This worked
when we only dealt with ata and ahci devices, but now we
support nvme devices so the logic is inherently flawed.
Instead, use grub_scan_disk to store the boot order, e.g.:
grub_scan_disk="ahci nvme ata"
grub_scan_disk="nvme ata"
In the first example, it would make GRUB scan ahci first,
then nvme and then ata.
In the secontd example, it would make GRUB scan nvme first,
and then ata.
If "both" is set, or anything other than ahci/ata/nvme,
grub_scan_disk is now changed to "nvme ahci ata".
Actual grub_scan_disk entries in target.cfg files will now
be modified, to match each machine.
Signed-off-by: Leah Rowe <leah@libreboot.org>
935447b035