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

script/netbootxyz-overrides.yml

@@ -8,7 +8,7 @@ generate_disks_rpi: false
 generate_version_file: true
 generate_local_vars: false
 bootloader_multiple: true
-bootloader_disks:
+bootloader_flavors:
   - "netboot.xyz"
   - "netboot.xyz-metal"
 generate_signatures: true