powerpc/booke: Change KERNBASE to be physical load address

Previous commits have made VM_MIN_KERNEL_ADDRESS its own separate entity,
and rebased the kernel around that address instead of KERNBASE. This commit
pulls the trigger to rebase KERNBASE to a physical load address. The
eventual goal is to align the address with the AIM KERNBASE, but at this
time that's not an option.

Currently a Book-E kernel must be loaded on a 64MB boundary, due to size
issues. The common load address is at the 64MB mark (0x04000000), so simply
make that the default KERNBASE.

As of this commit, Book-E kernels can be loaded and booted with ubldr.

MFC after: 3 weeks