PPC logics for choice of default boot device number switched to
dev_t.  Rationale: it belongs to userland; we notice partitions with
certain properties and choose one of them as the best candidate for
being a root fs; then we put the resulting device number into ROOT_DEV,
which has only one use - it's passed to mknod(2) to create a device node
on rootfs and allow mount(2) get us the final root.

	IOW, all that code has nothing whatsoever with kernel internals
of any description - the value we are generating will be passed to
mknod(2) anyway.  Switched to dev_t.

	Note: that code should eventually be moved to early userland.