PowerPC kernels in r6 is actually metadata from loader(8) or gibberish
left in r6, which is not required to be anything under the
PAPR/ePAPR/CHRP/OF standards, by another boot loader.
Note that, as a result, systems need a new boot loader to boot PPC kernels
after this revision without ending up at a mountroot prompt. New boot
loaders are backwards compatible and can boot older kernels.
Reviewed by: jhibbits
MFC after: 2 months