Use the cookie now set by loader to determine whether the value passed to
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