Switches bootloader_disks to bootloader_flavors
Ensures bootloader_disks is set to one type to keep
index generation bound to one falvor type. Shifts generate
disk loop to use bootloader_flavors variable taht can be stacked
with multiple types.
Should fix issue causing the index to have the last name in the list
of bootloader_disks which was setting the incorrect name. This was
probably introduced with the change in:
https://github.com/netbootxyz/netboot.xyz/pull/1096