Building a pure64 kernel using pure64 tools

[Alex Perry <alex.perry@qm.com>]

As far as I can tell, you need 32 bit support in the toolchain
if you want a "zImage" or a "bzImage" in order to compile the 
decompressor.  Can I override kernel-image's assumption of bzImage ?

Otherwise, the pure64 partition is not self-sustaining since its
kernels need to be built by a separate boot with a different toolchain.