discover/platform-powerpc: Reject bootdevs with empty UUIDs

If a "uuid:" label is set in the petitboot,bootdevs parameter without a
matching UUID, the UUID is unintentionally accepted and set to NULL.
This can cause a segfault in nc-config when device UUIDs are compared
against the autoboot option. Instead treat options like this as
malformed.

Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>